TORONTO -- If it wasn’t the most dominant pitching performance by a Mariner this season, it was certainly the most impressive.

However, Easton McGee’s 6 2/3 innings of scoreless ball in his first MLB start -- in which he didn’t allow a hit until the seventh -- was soured by a Seattle offense that came one strikeout shy of its franchise-worst record of 20, which led to a 1-0, 10-inning loss to the Blue Jays on Saturday at Rogers Centre.

This was a pitchers’ duel from start to finish.

Recalled from Triple-A Tacoma hours before the matinee, McGee tormented Toronto’s loaded lineup with endless weak contact thanks to a six-pitch mix, quality command, and above all, trust in his stuff. He carried a no-hitter into the seventh inning before surrendering a 406-foot double off the top of the center-field wall to Matt Chapman with two outs, which ended his day.

From the other dugout, Kevin Gausman racked up a career-high 13 strikeouts and generated 28 whiffs. He gave up six hits, but the Mariners were only able to reach scoring position against him four times -- including a first inning that saw Seattle put runners on first and third with one out, before Cal Raleigh and Teoscar Hernández struck out to end the threat.

The club's other opportunities ended with a pickoff of Tom Murphy at second base and strikeouts from Raleigh and Sam Haggerty.

“He's really good, we knew that,” Mariners manager Scott Servais said of Gausman. “Guys that are that good and that built out, you've got to get on them early. We had some chances there, in the first and second inning, and we weren't able to cash in.”

McGee made his first MLB start in his first appearance with the Mariners and just his second in MLB, having debuted last Oct. 2 in relief for Tampa Bay. He took the rotation spot -- at least temporarily -- of Chris Flexen, who had made four starts with a 10.58 ERA filling in for Robbie Ray after the lefty was shelved with a left flexor strain. Ray now needs season-ending surgery, which has cast uncertainty on how the Mariners will handle the role long term.

McGee remarkably threw only 64 pitches (45 strikes), and he was aided by strong defense, striking out two and limiting Toronto to just six hard-hit balls of the 19 put in play, with an 87.1 mph average exit velocity. He also walked Chapman in the fifth for his first baserunner after keeping each of Toronto’s first 12 hitters from reaching.

“I don't have much experience, obviously,” McGee said. “But I feel like you can't go out there and compete unless you're trusting yourself.”

What the Mariners do with the rotation spot is to be determined, especially given that Flexen seemed like a stable fill-in before experiencing a downtick in stuff and an uptick in hard contact. McGee made a strong case, but the Mariners also have interesting prospects, such as Bryce Miller (their No. 2-ranked by MLB Pipeline), Emerson Hancock (No. 4) and Bryan Woo (No. 6), all of whom figure to be a big part of the team’s future.

• Mariners' Top 30 Prospects list

McGee, a fourth-round pick in the 2016 MLB Draft, is unranked, and he was mostly unknown before Saturday. Acquired from Boston for cash considerations on Nov. 9, he labored through three outings in Spring Training, surrendering 12 hits and seven runs. Yet in five outings for Tacoma, McGee had a 3.14 ERA and limited hitters to a slash line of .241/.297/.343 (.639 OPS).

“That was one of the funnest games I’ve ever gotten to catch,” said Murphy, who caught McGee’s first Cactus League outing, when he gave up five runs in 1 2/3 innings. “Just being out there and seeing a kid that probably had no idea that he was going to be doing this a couple days ago just go out and dominate a very, very good lineup.”

McGee carved through a group that’s been one of the league’s best, and remarkably, he went toe-to-toe with Gausman. The Mariners’ starter came closer to flirting with history, but stuff-wise, Gausman was Saturday’s headliner, now leading MLB with 54 strikeouts in the season.

Gausman generated six K’s on his four-seamer and seven on the splitter, which the Mariners had no answer for, whiffing a whopping 20 times and going 1-for-10 against the late-breaking pitch.

In their 47-year history, the Mariners had only twice struck out more than the 19 times they did on Saturday: on Sept. 25, 2012, against the Angels, and on April 29, 1986, against the Red Sox, the game in which Roger Clemens became “The Rocket.” Seattle hitters have struck out in 25.6% of their plate appearances this year, better than only the Giants (27.5%) and Tigers (25.7%).