"I just like the grit in my club," Hurdle said. "Offensively, we scratched."
Juan Nicasio (6-6) struck out two in the ninth for the victory, then Mark Melancon allowed a pair of singles before finishing for his 23rd save as the Pirates won for the sixth time in eight games after dropping eight of nine.
Pittsburgh also has won the last three meetings against the A's after losing the initial 11.
A's reliever Ryan Madson had runners on second and third in the ninth following a double steal but retired Sean Rodriguez on a popup and induced Josh Harrison's groundout. The Pirates challenged whether Danny Valencia's throw from third to first was in time, but the call was confirmed in 1 minute, 55 seconds.
Jed Lowrie had a sacrifice fly and Josh Reddick an RBI single in the third for Oakland, one of his career-high matching four hits. His double in the fifth got stuck in the outfield wall and wasn't ruled a ground-rule double, preventing a run from scoring.
"It was very frustrating. Could have won the game," he said. "Over the years I've obviously seen (the hole) but never really paid attention to it. After it happened I went out there and it was literally a three-inch gap to put a baseball in. Never hit a hole in one before even in golf so I guess this is going to have to count."
Harrison hit an RBI double and Starling Marte a run-scoring single in the fourth as Pittsburgh tied the game at 2.
Mercer led off the game with a double on a ball that got lost in the sun and dropped in shallow right field between three A's players.
Rich Hill came off the disabled list to strike out six while pitching four-hit ball over six innings in his first outing since May 29 after being sidelined with a strained left groin. He had a five-start winning streak since losing May 1 to Houston.
Hill allowed two runs on four hits and walked two, limited to 83 pitches based on how much time he missed.
Pirates right-hander Chad Kuhl gave up seven hits and two runs, struck out two and didn't walk a batter in six innings. It was his second major league start after beating Clayton Kershaw and the Dodgers in his debut -- and he got the nerves and any awe factor out then.
"Now it wasn't my first time anymore," he said. "Just another day at the yard."
Crisp hit his 300th career double in the third and extended his hitting streak to nine games.
The A's ended a stretch of hitting a home run in a season-best 10 straight games, playing their first extra-inning game at home after 60 without one since Aug. 18 last year against the Dodgers.
Pirates: RHP Gerrit Cole (triceps strain) shadow threw in the bullpen a day after a simulated game. ... The Pirates have gone a week without a roster move. "Shhh, shhh," Hurdle said. "It's been welcome."
Athletics: Oakland agreed to terms with versatile, 17-year-old Cuban OF Lazaro Armenteros on Saturday. He receives a $3 million signing bonus. The A's also agreed to terms with Dominican players 3B George Bell, SS Marcos Brito, SS Yerdel Vargas, and CF Kevin Richards. All of the new acquisitions will report to the club's Dominican training facility and later to Arizona for instructional league this fall. "Organizationally, it's a big day," manager Bob Melvin said.
Pirates: In his last start against the A's, Sunday starter LHP Francisco Liriano(4-8, 5.33 ERA) pitched seven scoreless innings in a 5-0 win on July 10, 2013.
Athletics: RHP Daniel Mengden (1-3, 2.81) makes his fifth big league start and is coming off his first major league win last Monday at San Francisco. He is the eighth pitcher in Oakland history to allow three or fewer runs in each of his initial four starts.