The A's closer hasn't blown a save in his last 36 chances, but he got his first loss in almost a year when he gave up a three-run homer to pinch-hitter Kendrys Morales in the 10th inning of a 6-3 defeat to the Seattle Mariners on Sunday.
Balfour (0-1) came on to start the 10th with the game tied, a situation he's not used to.
"I wasn't expecting the call, I guess," Balfour said. "I guess I'm waiting for us to score to go in and save the game. So I didn't have the right mind frame going in maybe."
After getting a leadoff groundout, Balfour struck out Mike Zunino, only to have him reach first on a wild pitch.
"The first pitch to Morales and the next thing you know it's three runs," Balfour said. "It's like you struck the guy out and you've got 3-2—then base hit and it's one pitch, three runs. It happened pretty quick."
Oakland manager Bob Melvin said he was ready to use Balfour for two innings and wasn't going to wait for a different situation to bring out his closer once it went into the 10th.
"We had a couple guys who weren't available and he's our best pitcher," Melvin said. "We were going to go with our best guy going into the day off.
Oakland started the day in a tie with Texas for the American League West lead but has lost four of five. The bullpen has been an issue in the recent slide—before Sunday, the A's bullpen had blown three games. Relievers picked up three losses and only one save over the previous six games.
Raul Ibanez gave the Mariners the lead with a two-run homer in the first inning and added a solo shot in the third, his team-leading 17th, to give Seattle a 3-0 lead. It was his second multihomer game this season and 18th of his career.
Oliver Perez (2-1) pitched two innings to earn the win. Perez has not allowed a run in his last 11 innings.
Brandon Moss' solo home run off reliever Danny Farquhar in the eighth inning tied the game at 3.
Seattle starter Jeremy Bonderman, who had allowed two runs over his previous three starts, cruised until getting into a jam in the sixth as the Athletics loaded the bases with one out.
With a 2-2 count on Josh Donaldson, Bonderman threw a wild pitch that allowed Jed Lowrie to score. Bonderman walked Donaldson on the next pitch to load the bases again and was relieved by Farquhar.
Farquhar got Josh Reddick to groundout to second base, but Nick Franklin had to make a sliding stop and could only get the out at first as Yoenis Cespedes scored to make it 3-2. Farquhar then got out of the inning when pinch-hitter John Jaso grounded out.
It was the first appearance for Jaso since sustaining an abrasion on his palm when sliding Thursday. He stayed in the game at catcher, but said Zunino reaching on the wild pitch didn't have anything to do with the injury.
"I just messed up the block," he said. "It was my responsibility that I didn't do."
Oakland starter Jarrod Parker kept his four-game winning streak alive with a no-decision, giving up three runs on five hits with one walk and six strikeouts in six innings.
"He pitched well," Melvin said. "Over 100 pitches for him today, and that is what we needed to give us a chance to win."
The Athletics are now headed back to Oakland for an eight-game homestand starting Tuesday after a 2-5 road trip.
"Wasn't a very good one for us," Melvin said. "Not much we can do about it now. Just take the day off, recharge and come back out on Tuesday and play our best baseball."
NOTES: After the game Oakland optioned RHP Dan Staily to Triple-A Sacramento. ... Seattle CF Franklin Gutierrez, playing for the second straight game after coming off the 60-day DL, left the game after the sixth inning with a tight hamstring. ... CF Coco Crisp was the only Oakland starter that had previously faced Bonderman. Crisp is now 10 for 45 with a home run against the right-hander.