OAKLAND, Calif. -- Landon Powell hadn't hit right-handed in more than a month. It had been much longer than that since Oakland's backup catcher hit a home run.

Both streaks ended emphatically Sunday, and suddenly the Athletics are one of the hottest teams in the American League.

Powell hit a tiebreaking home run in the eighth inning, his first homer since September, and the A's held on to beat the San Francisco Giants 2-1, completing a three-game sweep at home of the defending World Series champs.

"Sometimes you're pitching has to do it, sometimes you're hitting has to do it," Powell said after Oakland extended its season-high winning streak to five games. "That's what good teams are about and that's what we're trying to be."

The A's were at the opposite end of the spectrum two weeks ago but have picked up steam since Bob Geren was fired on June 9 and replaced by Bob Melvin.

After dropping three of its first four games under Melvin, Oakland won two of three from Kansas City before pulling off the sweep of the Giants. The A's also pulled within five games of first-place Texas in the AL West heading into Monday's off day.

"When you play games like this you just want to keep rolling every day," Melvin said. "If you stay positive some good things can happen."

Powell, making a rare start in place of catcher Kurt Suzuki, was hitless in two at-bats against San Francisco starter Matt Cain before hitting his first homer of the season off reliever Jeremy Affeldt (1-1).

It was only Powell's 10th career homer.

"I hadn't hit right-handed in a month so I was just trying to simplify my swing, get my foot down early and be short to the ball," Powell said. "He came at me with fastballs and I was just able to get the fat part of the bat on it and drive it."

Powell's homer spoiled what had been a pitching duel between Trevor Cahill and Cain. It also gave Cahill a victory after he was on the hook for a no-decision.

Cahill (7-5) pitched eight innings to win for the first time since May 9. Oakland's right-hander, who was 0-5 in his previous seven starts, pitched out of jams in the second and third to help the A's extend their season-high winning streak to five.

Aaron Rowand had an RBI double for San Francisco, which wasted a stellar outing by Cain while losing its fourth straight game overall.

"We have to get this offense going," Giants manager Bruce Bochy said. "It's been like this for a while. It's frustrating for us. Guys just aren't having years they are supposed to have."

A's rookie Jemile Weeks had three hits, his fifth multi-hit game since being called up from the minors on June 7. That helped Oakland complete a 5-1 homestand, the team's best since 2006.

Brian Fuentes pitched a 1-2-3 ninth for his 12th save in 15 chances. Fuentes struck out Cody Ross looking to end the game, and the Giants outfielder angrily reacted.

Cahill scattered five hits and had seven strikeouts. He had been 0-5 with a 5.67 ERA in his previous seven starts before putting the clamps on the Giants.

"Before, I was yanking a lot of pitches, and instead of throwing that sinker away it was just kind of cutting," Cahill said. "Today I kind of got the feeling for it coming back over the plate to righties and inside to lefties. I just think the command was there more than in the last couple of starts."

Cain was just as tough but left with his fifth no-decision of the season, ending a three-game winning streak.

Cain gave up a leadoff single to Weeks in the first -- the only hit Oakland got until Weeks doubled with one out in the sixth. In between, San Francisco's right-hander retired 14 straight during one stretch, including designated hitter Hideki Matsui, who twice popped out weakly to second.

The A's broke through after Weeks' double, however. The speedy Weeks, called up from the minors on June 7, moved to third on Cliff Pennington's groundout and scored on Coco Crisp's single to right.

Cain allowed only three hits, had five strikeouts and didn't walk a batter for only the second time this season. But with the Giants' offense still sputtering with runners in scoring position -- they are 0 for 26 over the last four games -- it wasn't enough.

Weeks, who had two of the hits off Cain, has been a big lift to Oakland's lineup. He batted leadoff for the second straight day and raised his average to .364, leaving the A's in a potential quandary when second baseman Mark Ellis is eligible to come off the disabled list next week. Melvin said prior to the game that the situation will "work itself out."

Game notes
Cahill's pickoff of Aubrey Huff in the second inning was the A's 15th of the season, tops in the majors. ... All three games in the series were sold out, giving Oakland five for the season. ... Giants IF Mike Fontenot has been slow to recover from a groin strain and might not begin a rehab assignment until next week. ... Oakland RHP Rich Harden (strained right shoulder) will make his first rehab start for Triple-A Sacramento on Monday.