Fulmer scattered three hits over 7 2/3 scoreless innings with three strikeouts and one walk.
"It felt good," Fulmer said. "Early on I had a little trouble getting loose but once I got loose and settled in, everything kind of flew by. I was happy with the way it went."
Fulmer's outing was the longest of his six career starts. It helped Detroit give its bullpen a needed break and led the Tigers to their ninth win in 11 games after going 2-11 to start May.
"He threw well," Detroit bench coach Gene Lamont said. "He's got quite a few weapons. Maybe didn't use the changeup quite as much but he's got an awfully good sinker. When he's throwing his fastball and it's down, he's going to get more ground balls."
Fulmer had 11 strikeouts in his win over the Rays on May 21 but took a different approach against the A's. That meant fewer changeups and a steady diet of fastballs down in the zone.
"I want to be more efficient and I want to go deeper into games, so that means throwing more quality fastballs and getting weak contact," Fulmer said. "Keeping them honest with the changeup and the slider was OK tonight, but I think the four-seam fastball down in the zone [was key] and just let them hit so the defense can work."
Fulmer (4-1) has won three decisions since losing to Washington on May 5. Over his last two starts, he's allowed one run and seven hits over 14 2/3 innings while lowering his ERA from 6.52 to a respectable 3.97.
"He has a great pitching mentality," second baseman Ian Kinsler said. "He wants to throw nine innings."
It's possible that Fulmer could still be the odd man out when the Tigers' rotation gets healthy. Jordan Zimmermann could return next week after missing his scheduled start Saturday, and Shane Greene is also close to coming back off the disabled list.
"I'm still learning," Fulmer said. "I feel so much more comfortable. I just trust everything. It feels good to get big league hitters out any way I can."
Lamont, noting that the decision isn't entirely up to him, said he thought the rookie pitcher has done enough to stay with the big league club after being acquired from the Mets on July 31 as part of the trade for slugger Yoenis Cespedes.
"We paid dearly for him," Lamont said. "I'm not knocking the kid but he came in a pretty big trade for Cespedes. You expect to get something good for Cespedes and he's done a good job for us so far. I think he's shown enough that he's going to stay here for a while."