OAKLAND, Calif. -- When shortstop Jordy Mercer stepped to the plate with one out in the third inning Friday night, the Pittsburgh Pirates trailed by three runs and hadn't put a runner on base against Oakland Athletics right-hander Sonny Gray.

Four pitches later, Mercer lined Gray's fastball over the left field fence, and the Pirates comeback was officially ignited. Pittsburgh scored seven unanswered runs in a 7-3 victory against the A's at the Oakland Coliseum.

Matt Joyce, Josh Harrison and Mercer -- the Pirates sixth, seventh and eighth hitters -- each had two hits and combined for five RBIs and four runs.

"We just stayed the course," Harrison said. "They got a couple innings early on, but we knew we were still within striking distance. That's the name of the game, chip away, and it started with Jordy's home run. You could kind of feel from that point like we were going to get him. Just put put together some good at-bats as a team. It wasn't that easy on him."

The Pirates scored another run in the fourth inning, two more in the fifth and three more in the sixth after Gray retired the first two batters. Joyce had an RBI single and Harrison a two-run double in the fifth.

"Joyce had a nice game, had some some big hits," Pirates manager Clint Hurdle said. "Harrison again with a big swing, Mercer with a couple RBIs. Good stuff down there at the bottom from 6, 7 and 8."

The Pirates beat the A's in Oakland for the first time in seven tries and improved to 2-11 overall in the interleague series.

Pirates left-hander Jeff Locke (8-5) allowed three runs on three hits over five innings and extended his winning streak to three games. He struck out three, walked three and threw 95 pitches.

In his previous two starts, Locke went 2-0 with a 0.66 ERA, six strikeouts and zero walks. He wasn't nearly that sharp against Oakland, but blanked the A's for his final three innings.

"You can't quit out there," Locke said. "There's definitely been times in my career where I felt like I quit. Now that I'm older and matured a little bit more, you got to keep going until the manager comes and takes it from you, because the guys behind you aren't going to quit. You got to keep going. You want to be someone that can be relied on."

Gray (3-7) lost his career-high sixth consecutive game, and hasn't won since April 22 at Toronto. Gray gave up a season-high-tying seven runs on seven hits over six innings. He struck out four, walked one, hit a batter and threw two wild pitches, giving him a major-league-high 13.

"That was obviously not a very good start," Gray said. "There were times, like in the sixth, where I got two quick outs. I have to be able to shut that inning down and get back out there for the seventh. And not hit a guy and walk another. It just escalated from there."

Marcus Semien hit a two-run homer, and Jed Lowrie had an RBI double and scored a run for the A's.

A.J. Schugel threw a scoreless sixth inning for the Pirates, and Arquimedes Caminero pitched two perfect innings, striking out the last four A's he faced. Mark Melancon pitched a perfect ninth, extending the bullpen's streak of scoreless innings to 24 2/3.

Semien gave Oakland a 2-0 lead in the first inning, hammering a home run after Lowrie worked a leadoff walk. Semien's home run was his 17th of the season -- an ongoing career high -- and second in two days. Semien crushed Locke's 3-2 pitch over the left-center-field fence.

"He keeps getting better and better and the production goes up," A's manager Bob Melvin said of Semien.

The A's extended their lead to 3-0 in the second inning. With two outs, Yonder Alonso beat the shift with a bunt single down the third base, and Lowrie brought him home with a double off the left field wall.

Mercer cut Oakland's lead to 3-1 in the third with his fifth homer of the season.

"I was able to get something on the board and then we kept it rolling after that," Mercer said.

The Pirates made it 3-2 in the fourth when Andrew McCutchen lined a double with one out, moved to third on a ground out and scored on Gray's 12th wild pitch of the season.

Joyce and Harrison opened the fifth inning with back-to-back singles, and Mercer singled Joyce home. Harrison stole third and scored when Chris Stewart grounded into a force out.

NOTES: The Athletics optioned LHP Dillon Overton to Triple-A Nashville and recalled INF/OF Tyler Ladendorf on Friday. ... Pirates RHP Gerrit Cole (right triceps strain) pitched a simulated game Friday in Oakland. Cole was placed on the 15-day disabled list June 14, retroactive to June 11. Cole said he went through his normal pregame routine then threw 45-to-50 pitches over two innings. ... Pirates RHP Ryan Vogelsong (facial fractures) pitched a simulated game. He has been on the 60-day disabled list since May 24. ... A's OF Jake Smolinski made his first career major league start and appearance in center field. ... A's LF Khris Davis returned to the starting lineup after missing two games with a bruised left shoulder and went 0-for-4. ... Pirates RF Gregory Polanco (sore left leg) made his first start in the outfield since June 23. Polanco was the Pirates' designated hitter the previous two games. ... The A's, according to multiple reports, are expected to sign 17-year-old Cuban OF Lazaro Armenteros on Saturday when the international signing period begins.