Given his decreased fastball velocity, Mike Fiers was aware of how slim the margin for error was going to be against a potent Blue Jays offense. After carefully navigating through the tough lineup early on, the right-hander began to falter.

Twice through the first three innings, Fiers was provided a one-run lead. Twice, he was unable to make the early advantages hold up, allowing five runs on nine hits -- including two homers -- and two walks over just 3 1/3 innings in Thursday’s 10-4 loss to Toronto at the Oakland Coliseum.

Even as Fiers got through a scoreless first two frames on 20 pitches, there were warning signs. His fastball -- which averaged 86.6 mph on the day -- was getting tagged; four Blue Jays hitters registered an exit velocity of at least 99.3 mph through those two innings.

Just two batters into the fourth -- shortly after the A’s regained the lead at 4-3 on Matt Olson’s ground-rule double and a two-run single by Sean Murphy in the third -- Fiers allowed his fourth and fifth runs of the day on a two-run homer surrendered to ninth-place hitter Danny Jansen. Fiers would record just one more out before getting pulled after yielding a single to Bo Bichette.

“They made it tough on me,” Fiers said. “They were very aggressive. I was just giving them too good of pitches early on and not expanding when I got two strikes. That lineup is really good. When you pitch like that and don’t stick to the game plan, you’re going to get hurt.”

The Blue Jays clearly came out with a plan to try to jump on Fiers early in the count. Of his 20 batters faced, 13 concluded their plate appearance within the first three pitches. In what was his second start of the 2021 campaign after he began the regular season on the injured list with a lumbar strain, Thursday’s outing was the 35-year-old righty’s shortest since Sept. 14, 2019.

Despite Fiers’ early exit, the A’s were well within striking distance. But for a second straight day, Oakland’s bullpen had a tough time keeping the Blue Jays’ offense down.

Facing a one-run deficit with two on and one out in the sixth, right-hander Deolis Guerra intervened on a grounder up the middle by Vladimir Guerrero Jr. that appeared to have a good shot at turning into an inning-ending double play. Picking the ball up, Guerra fired a wide throw to second base that instead loaded the bases and led to an eventual four-run inning to break the game open.

“You get a couple outs there and it puts us in a position to make a comeback,” Murphy said. “But you look at the rest of the game, we didn’t score either. It may or may not have made a difference. [It was] one of those plays where you look back at what could have been.”

A’s relievers combined to allow five runs (four earned) over the final 5 2/3 innings. Splitting the four-game series, the Blue Jays really turned it up against A’s pitching over the final two games, scoring 19 runs in those contests after being held to just five runs in the opening two games. Of those 19 runs, 12 were allowed by the bullpen.

“They pound the ball pretty good,” A’s manager Bob Melvin said. “You better be locating pretty well and making some good pitches. For the first two games, we were pretty good at that up until the end of [Wednesday’s] game. Today, they broke through on us.”

Toronto’s final run came off the bat of former A’s shortstop Marcus Semien, who sent a solo shot over the Coliseum’s left-field wall in the seventh to cap a 4-for-6 day.

Semien -- who spent six seasons with the A’s from 2015-20 and received a standing ovation before his first plate appearance of the series Monday night -- was cheered by the Oakland faithful as he rounded the bases.

“He knows our pitchers,” Murphy said. “It’s sort of a chess match with a guy like that. We tried to keep him guessing, but he obviously had a very good day.”