SEATTLE -- Can the Mariners jumpstart what they hope and believe will be a big step forward toward contending in 2021 after a promising third-place finish last season? Will the Giants begin establishing themselves in a top-heavy, loaded National League West?

We’ll find out Thursday, when San Francisco visits Seattle for a rare Interleague matchup on Opening Day at T-Mobile Park.

• What to know as fans return to T-Mobile Park

Both clubs enter 2021 far more optimistic about their rosters -- and immediate and long-term futures -- than they have in at least three years. The Mariners have the No. 3 farm system in all of baseball, per MLB Pipeline, and many of those prospects will contribute in 2021. The Giants fell one win shy of reaching the postseason last year, and their veteran core of Buster Posey, Brandon Crawford, Brandon Belt and Johnny Cueto wants to make one more run as each player enters a contract year.

These clubs squared off four times last season as part of the revised schedule that pitted all West division teams against each other, with the Giants winning all four games. Here is a preview of the matchup on Thursday, which will launch each team's 162-game 2021 season:

When is the game and how can I watch it?
First pitch is slated for 7:10 p.m. PT on Thursday and will be available on MLB.TV. For Mariners fans, the game will be televised on ROOT Sports Northwest and on the radio on 710 ESPN. For Giants fans, the game will be televised on NBC Sports California, with KNBR providing the radio broadcast.

The starting lineups
Giants: Newcomer Tommy La Stella frequently led off for the Giants during Cactus League action, but he’s unlikely to start on Thursday since the Mariners are planning to send a left-handed pitcher to the mound in Marco Gonzales. Expect the Giants to instead stack the top of their lineup with right-handed bats like Austin Slater, Donovan Solano, Evan Longoria and Wilmer Flores, who helped the club post an .838 OPS against left-handed pitchers last season, which ranked fourth in the Majors. With little turnover among their position player group, the Giants should match up well against lefties again in 2021.

Another notable development will be the return of Posey, who will be back behind the plate after sitting out the 2020 season. The 2012 National League MVP Award winner and six-time All-Star will likely hit lower in the order than he has in the past, which is a sign of the depth of the Giants’ lineup.

Lineup

1. Austin Slater, LF
2. Mike Yastrzemski, RF
3. Donovan Solano, 2B
4. Evan Longoria, 3B
5. Brandon Belt, 1B
6. Wilmer Flores, DH
7. Buster Posey, C
8. Brandon Crawford, SS
9. Mauricio Dubón, CF

Mariners: The Mariners will be without everyday cleanup hitter Kyle Lewis after the reigning AL Rookie of the Year was placed on the 10-day IL on Wednesday with a bone bruise in his right knee. So, Evan White will bring his team-high 16 RBIs from Spring Training into the four-hole and bat behind a much improved one-through-three to set the tone. Mitch Haniger was a force after moving to leadoff for his final 13 spring games, and he had huge success in that role the last time the Mariners used him there regularly in 2018. There’s also a lot to like about Ty France, who was by far Seattle’s best hitter this spring, hitting second. Kyle Seager also remains an above-average lefty bat after posting a 121 OPS+ in ’20, and he’ll once again be leaned on as a key run producer -- particularly with Lewis sidelined.

But beyond that group, the production potential dips in the lower half of the order. Tom Murphy led the Mariners with 3.2 wins above replacement in 2019, but he hit just .200/.289/.375 this spring and missed all of ’20 with a foot injury. Taylor Trammell was nearly as impressive as France at camp, but he brings an element of the unknown given that this will be his first bit of big league action. J.P. Crawford lost his grip on the leadoff role after notching just five hits in 50 spring plate appearances.

Lineup

1. Mitch Haniger, RF
2. Ty France, DH
3. Kyle Seager, 3B
4. Evan White, 1B
5. Taylor Trammell, CF
6. Dylan Moore, 2B
7. Jake Fraley, LF
8. Tom Murphy, C
9. J.P. Crawford, SS

Who are the starting pitchers?
Giants: Kevin Gausman earned the Opening Day assignment after emerging as the Giants’ best starter in 2020, when he logged a 3.62 ERA with 79 strikeouts over 59 2/3 innings. The 30-year-old right-hander was also tapped to start the Orioles’ opener in ‘17 and delivered 5 1/3 innings of two-run ball in a 3-2 win over the Tigers.

Gausman reportedly received multiyear offers from other teams over the offseason, but he chose to return to the Giants on a one-year, $18.9 million qualifying offer and has praised the organization for helping him get back on track after struggling with the Braves and Reds in 2019. He pairs his mid-90s fastball with a devastating splitter that induced a 49 percent whiff rate last year, per Statcast.

“It's nasty,” Giants right-hander Logan Webb said. “I think it's one of the best pitches in baseball, to be honest with you. I would say it's the best splitter in baseball.”

Mariners: Gonzales will take the hill for the regular-season opener for the third straight year, which in Mariners history puts him behind only Jamie Moyer (four), Randy Johnson (six) and Félix Hernández (11), whose streak was snapped in 2019 when Gonzales’ current run began. The cerebral, location-driven left-hander takes the opportunity as “a true honor to be mentioned in the same sentence as some of those names.” Gonzales has made just two starts against San Francisco in his career, most recently on April 3, 2019, when he threw 6 1/3 innings and gave up three runs, including two homers, in a 6-4 Seattle win.

• What’s next for unquestioned leader Marco in 2021?

How might the bullpens line up after the starter?
Giants: Giants manager Gabe Kapler has refrained from naming an official closer and will likely mix and match to start the season, but veteran left-hander Jake McGee appears to be a strong candidate to pitch the ninth inning after joining the club on a two-year, $5 million deal over the offseason. McGee, 34, has 45 career saves in the Majors and looked fantastic this spring, firing 6 1/3 scoreless innings with eight strikeouts and no walks. Matt Wisler, Tyler Rogers and José Álvarez are also expected to be key bullpen pieces and will likely be trusted to handle high-leverage situations during the regular season.

Mariners: If things go according to plan for the uber-competitive Gonzales, Seattle won’t have to turn to its bullpen until the sixth or seventh inning on Thursday. If that’s the case, Servais would be happy to rely only on setup man Kendall Graveman and closer Rafael Montero. But things can change quickly, and the Mariners -- whose 5.92 team ERA last season was fourth-worst in the American League -- believe they have an improved group. Starter Nick Margevicius broke camp as the longman to give Seattle more innings insurance in the early leg of the season, Casey Sadler has looked decent and Domingo Tapia could be nearing a return from an oblique injury. Add Keynan Middleton and Will Vest in the mix, and Seattle’s relief corps has over four arms that average more than 95 mph on fastballs after posting the second-slowest velocity last season.

Any injuries of note?
Giants: Left-hander Alex Wood is likely to open the season on the injured list after undergoing an ablation procedure to treat stiffness in his back earlier this month. First baseman Brandon Belt is coming off right heel surgery and missed the beginning of camp with a bout of mononucleosis, but he progressed rapidly in recent weeks, stoking optimism that he’ll be ready to go for Thursday.

Mariners: The Mariners had a mostly clean bill of health all spring until Lewis collided with the outfield wall on March 22 and has not been able to work out since. Servais said on Wednesday that Lewis' recovery will be a matter of weeks. Additionally, second baseman Shed Long Jr. has been sidelined from games all spring after his season-ending shin surgery last September has taken longer to heal than he or the club anticipated. Reliever Roenis Elías had been a lock to secure a spot in the bullpen, but he tore his UCL in camp and will need Tommy John surgery.

Who's hot and who's not?
Giants: Most of the Giants’ regular position players had nice springs at the plate, though Solano looked especially impressive after hitting .421 with a 1.187 OPS over 15 exhibition games. After capturing his first career Silver Slugger Award at second base last year, Solano has said that his goal is to hit for more power in 2021. He appears well on his way to achieving that aim, as he hit three home runs this spring to match his total over 54 games last year.

Webb enjoyed arguably the best spring of any Giants player, posting a 0.53 ERA over five exhibition starts to secure a spot in the starting rotation. He is expected to start Game 3 on Saturday and will enter his third Major League season with plenty of confidence after allowing only one run, seven hits, two walks and 22 strikeouts over 17 innings this spring. Conversely, Cueto posted a 9.82 ERA over three Cactus League starts, though the Giants likely put little stock in those results given the 35-year-old’s wealth of experience and savviness on the mound.

Mariners: There weren’t many MLB hitters who had a better spring than France, whose 1.135 OPS was the sixth-highest of all players, and his five homers led Seattle. The Mariners have raved about France since acquiring him from the Padres at last year’s Trade Deadline, and they even fielded offers from other clubs that tried to sway Seattle to immediately flip him. But the eye test has met the stat line so far, and France has emerged as not only an elite hitter, but also a long-term foundational piece to the rebuild. Also acquired in that trade, Trammell has had just as impressive of a spring and essentially played his way onto the roster.

Conversely, Crawford hit .128/.346/.128 in 18 spring games, with more strikeouts (eight) than hits (five). He looks much stronger, but even he recognizes that his offensive profile could use more extra-base hits.

Anything else fans might want to know?
• Thursday’s game is a sellout of the 9,000-fan capacity that the Mariners have received clearance for from the State of Washington and King County for the first two homestands, spanning 11 games. Those protocols could change with more health and safety developments.

• The Giants have had 13 different players start in left field on Opening Day since Barry Bonds played his final season with the club in 2007, and that streak appears likely to remain alive this season. Alex Dickerson started in left for the Giants in 2020, but Slater is likely to get the start there with Gonzales on the mound this year.