SAN FRANCISCO -- Last week, rookie Sammy Long noted that he had checked off most of the big milestones over his first three Major League outings: first appearance, first start, first win.

“I’m knocking these firsts out of the way pretty swiftly,” Long said after defeating the Phillies on Father’s Day. “It feels good to get the first win. I gave up the first bomb, so thank goodness that’s out of the way. It’s been a pretty action-packed first three outings.”

The action continued in the 25-year-old left-hander’s fourth big league appearance on Sunday afternoon at Oracle Park, which marked his first foray into the Bay Bridge Series. But Long couldn't overcome early command issues, and the Giants' bats fell flat in a 6-2 loss to the A's that denied them a three-game sweep.

The Giants entered Sunday averaging 7.36 runs per game over their previous 11 contests, but they were subdued by A’s left-hander Cole Irvin, who allowed only three hits and struck out eight over eight scoreless innings. Irvin became only the second starter to complete at least eight innings against San Francisco this season, joining the Nationals’ Joe Ross.

San Francisco showed late life in the ninth, scoring a pair of runs against reliever Deolis Guerra, but Lou Trivino struck out pinch-hitter Brandon Crawford to end the game and snap the Giants’ five-game winning streak.

“I liked the way the guys fought,” manager Gabe Kapler said. “We're better than we showed in the first couple of innings, but Irvin deserves a lot of credit for pitching a great game.”

Long, who has been filling in for the injured Logan Webb in the Giants’ rotation, fired six innings of two-run ball in his last start against the Phillies, but he endured a shaky start to his outing against the A’s. Long left a few pitches up against the top of Oakland’s lineup in the first inning, resulting in three consecutive singles that put the Giants in a quick 2-0 hole.

Opposing batters entered Sunday 0-for-18 against Long’s looping curveball this season, but the Fair Oaks, Calif., native misplaced the second one he threw to Elvis Andrus, who shot it to left field for a single. Matt Olson followed with another single, and Andrus and Olson then executed a double-steal to put a pair of runners in scoring position for Matt Chapman, who knocked them both in with a base hit to right-center.

“I think the first inning, I left a few pitches up there when I was ahead in the count that I wanted down,” Long said. “The curveball to Andrus was hung a little bit. I wanted to bury that one. Overall, I left some good pitches over the plate for them to hit. I think the adjustment after that was just trust the stuff and keep pounding the zone.”

Long settled in and posted four scoreless innings before running into trouble again in the sixth. He hit Chad Pinder to open the frame and then walked Tony Kemp to put a pair of runners aboard with no outs, prompting Kapler to summon right-hander John Brebbia from the bullpen. Brebbia surrendered only one hit -- an RBI single to former Giants catcher Aramis Garcia -- but the A’s managed to scratch across three runs to extend their lead to 5-0.

Long was charged with his first career loss after allowing four runs on four hits over five innings, raising his ERA to 4.95 on the season.

“I thought it was solid,” Kapler said of the rookie's performance. “I think after the first inning, he got right back in the zone and started to get a little bit more control of curveball and featuring it a little more. The fastball didn’t have necessarily the best life that it’s had in some previous outings. I think in the first inning he had difficulty finding his best stuff, but I thought he got better as the day went on. I thought it was a fine outing for us.”

Long, who grew up rooting for the Giants, said he relished the opportunity to pitch in his first Bay Bridge Series after watching plenty of Giants-A’s games from the stands throughout his childhood.

“The crowd was electric,” Long said. “To me, it looked like the biggest crowd of the weekend. The Bay series was one that I grew up looking forward to. To be on the mound for one of those games, that was awesome. It didn’t go my way this time. I’m happy we won the series, but it’s on to the next one.”

Despite the loss, the Giants (50-27) remain in possession of the best record in the Majors after becoming the first team to reach 50 wins this season, a feat they last accomplished in 1938. After wrapping up their local clash with the A’s, the Giants will now head to Los Angeles for another rivalry series against the Dodgers, who are four games behind San Francisco in the NL West, pending the outcome of their game against the Cubs on Sunday night.

The third Giants-Dodgers showdown will feature a pair of marquee pitching matchups, as Anthony DeSclafani will oppose Trevor Bauer in Monday’s series opener, and Kevin Gausman will battle Walker Buehler on Tuesday.

“We faced those guys twice already this year,” Buster Posey said. “You know you’re going to have a battle on your hands. We’re going to try to have a good approach and do what we can to try to come out on top.”