OAKLAND, Calif. -- Left-hander Francisco Liriano got back on track, and the Pittsburgh Pirates completed a three-game sweep of the Oakland A's with their third straight come-from-behind victory Sunday.

Liriano gave up three runs over five innings and ended his five-game losing streak with a 6-3 win against the A's at the Oakland Coliseum. Liriano (5-8) allowed six hits and struck out only two, but he walked just one batter, his lowest total since walking none on April 30 against Cincinnati. He won for the first time since beating Arizona on May 24. Over his previous six starts, Liriano went 0-5 with a 7.12 ERA and 24 walks in 30 1/3 innings.

"I was able to repeat my delivery better today," Liriano said. "It was down in the zone. ... I never lost my confidence. You go through tough times. I'm going through one right now. Just try to keep battling and try to get better every start."

Pirates manager Clint Hurdle considered Liriano's performance a big step in the right direction.

"One walk speaks volumes," said Hurdle, who was tossed out of the game by home plate umpire Sam Holbrook in the bottom of the sixth inning, apparently for arguing balls and strikes from the dugout. "You have to throw strikes to win in this league. You have to throw strikes to have success as a starting pitcher. He kept away from the big inning. I thought he stayed focused on making next pitch, especially when they did have men on base."

Rookie left fielder Adam Frazier made his second career start and went 2-for-4 with a triple and a double for the Pirates. He drove in the game-tying run with a sixth-inning triple then scored the go-ahead run that inning, giving the Pirates a 4-3 lead.

David Freese hit a two-run homer -- his eighth blast of the year -- in the eighth inning off A's reliever John Axford, extending Pittsburgh's lead to 6-3. Gregory Polanco went 2-for-4 and drove in two runs.

The Pirates (41-41) won their fourth straight game overall and climbed back to .500 for the first time since June 16. They went 4-1 on their road trip and swept Oakland for the first time.

"It's huge, especially to come from behind three games in a row on the road," Frazier said. "It's pretty big. We got the split in Seattle, but to get three come-from-behind wins like this, it's huge momentum heading back east to St. Louis."

A's rookie right-hander Daniel Mengden (1-4) allowed four runs on seven hits over 5 1/3 innings. Mengden, who was coming off his first career big league win, struck out five and walked four.

"He's pitched so well for us," A's manager Bob Melvin said of Mengden. "Every time out, he's pitched really well and deep in games. This is the first time he really hasn't. I don't think he was tired. They made some pretty good swings toward the end of his outing."

Coco Crisp and Khris Davis each had two hits for the A's. Billy Butler and catcher Matt McBride both doubled.

Pirates right-hander Mark Melancon pitched a perfect ninth for his 24th save overall and 20th straight. The Pirates' bullpen had four scoreless innings, extending its streak to 32 2/3 innings.

For the third straight game, Oakland scored first, this time taking a 2-0 lead in the second inning. Davis lined a one-out single and scored on Butler's double down the left field line. Butler moved to third on Jake Smolinski's sharp single and scored on McBride's ground out.

Pittsburgh pulled even with two runs in the fifth. Frazier lined a leadoff double to center, and Jody Mercer walked. Erik Kratz tried to advance the runners, but his bunt was too hard, and Mengden forced Frazier at third base.

Mengden hit John Jaso with a pitch, loading the bases, and Polanco lined a two-run single to center.

"Early innings I was getting pitches where I wanted too," Mengden said. "I was getting ahead. I feel like toward the end of the fifth and sixth, I started falling behind, not getting first-pitch strikes. I ran into trouble and they scored some runs. I think my stuff was fine. I just wasn't executing when I needed to. I left some pitches up."

Oakland responded with a run in the bottom of the fifth. McBride lined a leadoff double down the left field line and scored on Crisp's single with one out.

Pittsburgh regained the lead at 4-3 with two runs in the sixth. Matt Joyce led off with a walk, and Frazier lined an RBI triple down the right field line with one out. Mercer walked, and Kratz brought Frazier home with an infield single.

"We've done this before so it's not like we're doing anything new," Hurdle said of the comeback victory. "Guys don't quit. We got in a situation where we kept scratching against good pitching. Their kid threw real well, threw strikes, attacked the zone, mixed three pitches. Liriano got in a good place, more consistent with his delivery. The bullpen bent but didn't break. Had some traffic on and guys worked through it."

The A's had runners on first and third with one out in the sixth against reliever A.J. Schugel, but pinch hitter Stephen Vogt lined into a double play. The A's had runners on first and third with no outs in the seventh, but Neftali Feliz retired the next three batters in order, striking out Marcus Semien and Danny Valencia.

NOTES: A's RHP Fernando Rodriguez left the game with a strained right shoulder in the eighth inning after his third pitch to the second batter he faced. Manager Bob Melvin said Rodriguez will have an MRI and could land on the disabled list. ... A's C Josh Phegley (strained right knee) was placed on the 15-day disabled list Sunday, and "there's a potential" for surgery, Melvin said. Phegley had an MRI, and A's doctors are "mulling over what they're seeing right now in there," Melvin said. ... The A's recalled C Matt McBride from Triple-A Nashville, and he started against Pittsburgh. ... Pirates CF Andrew McCutchen was out of the starting lineup Sunday to rest a sore heel, manager Clint Hurdle said. "I think he actually banged his heel (Friday) night on the play at the plate. He managed it. He can manage it. I wanted to give him a blow today because I thought it was barking on him last night at the end of the game. Hopefully it will give him a little breather for our series in St. Louis." .. The Pirates claimed C Eric Fryer off waivers from St. Louis.