OAKLAND, Calif. -- The Texas Rangers made sure they'd have plenty of momentum for their crucial three-game series against Houston, which begins Friday at Minute Maid Park.

Left-hander Cole Hamels pitched six solid innings, and the Rangers cruised to an 8-1 victory against the Oakland Athletics on Thursday afternoon at O.co Coliseum, completing a three-game series sweep.

The Rangers increased their lead in the American League West over the idle Astros to a season-high 3 1/2 games.

"They've got some great energy right now, superior focus," Rangers manager Jeff Banister said. "They're on a mission. We've been talking about this. I believe that each one of them knows where we want to go, but we're not there yet, so they've maintained that focus."

Hamels, who came to the Rangers from Philadelphia on July 31 in an eight-player trade, won his fifth straight game, allowing one unearned run on four hits, striking out two and walking three. He improved to 5-1 in 10 starts with the Rangers and 11-8 overall this season.

The Rangers turned four double plays for Hamels, who had Athletics hitters pounding his pitches into the ground. Texas had a double play in the first, third, fourth and fifth innings.

"I'm not normally a ground ball pitcher, but it definitely helped out today," Hamels said. "Being able to get the big double plays in the key situations, it definitely kept them from rallying.

"What's coming next is going to be a tough series because Houston's at home. Everybody kind of knows what's in store there. We just have to keep bearing down and being ourselves and going out there and feeling confident and knowing we're going to get good games pitched by our starters and the hitters are going to go up there and score runs for us. It's exciting to be able to take this and move forward for tomorrow."

Rangers designated hitter Prince Fielder hit his 22nd home run of the season, a two-run blast in the eighth inning.

Third baseman Adrian Beltre went 2-for-5 and drove in three runs for the Rangers. Center fielder Delino DeShields had three hits in five at-bats, drove in two runs and scored twice.

Athletics designated hitter Billy Butler recorded his 1,400th career hit, a leadoff double in the seventh inning. Right fielder Josh Reddick went 1-for-2, stole two bases and scored a run.

Oakland right-hander Chris Bassitt made his first start since Aug. 26 after recovering from a sore shoulder. He gave up three runs on six hits over three innings and left after throwing 73 pitches.

"I felt good," Bassitt said. "Maybe a little rusty and I got the ball up a little bit. I got a lot of weak fly balls that turned into hits. That happens."

The Athletics (64-89) lost their fifth straight game and fell a season-high 25 games under .500.

"They have good team over there," Athletics manager Bob Melvin said. "They have great leadership and everybody is healthy. They have a lot of guys who've been there before. They're talented, and they brought in Hamels and made some pick-ups."

The Rangers (83-69) took a 2-0 lead in the first inning. DeShields hit a leadoff single, stole second base and scored on right fielder Shin-Soo Choo's double off the right-center field wall.

Choo moved to third on Fielder's ground out and scored when Beltre scorched a single to left past third baseman Danny Valencia.

Texas added a run in the second. Second baseman Rougned Odor led off with a single, moved to second on a ground out and to third on catcher Chris Gimenez's bloop single. DeShields brought Odor home with a single to center.

Oakland scored an unearned run in the second.

The Rangers scored three runs in the sixth. Odor lined a leadoff single and moved to second on a sacrifice bunt. After Gimenez walked, Odor scored on DeShields' sharp single to left. Fielder walked with two outs to load the bases and Beltre hit a two-run single.

Fielder made it 8-1 in the eighth, lining right-hander Cody Martin's 0-1 curve into the right-field seats after Choo walked.

"We know their season is not at the same level that we are right now, and we know that they don't have the team that they had at the beginning," Beltre said of the Athletics. "We came in trying to this, a least win the series and hopefully get a sweep because we're going to need every win that we can get right now, especially going into Houston."

NOTES: Oakland C Stephen Vogt was named the 2015 Jim "Catfish" Hunter Award winner for the second straight season in a vote by his teammates and coaches. The award honors a player whose performance on the field and conduct in the clubhouse exemplifies the late Hunter's courage, leadership and competitive spirit. ... Rangers OF Josh Hamilton (sore left knee) responded well to his heavy pregame workload Wednesday, when he ran the bases and took live batting practice. "He feels good," Rangers manager Jeff Banister said. "He's back out there this morning getting some defensive work in." For now, Hamilton is limited to pinch hitting. ... Athletics CF Billy Burns' fifth-inning triple Wednesday night was the team's 41st of the season, setting an Oakland record. It broke the previous mark of 40 set in 1968, the Athletics first year in Oakland. ... Rangers RHP Colby Lewis, who won his 17th game Wednesday, is in a three-way tie for second in the American League in victories, one behind Houston's Dallas Keuchel and Seattle's Felix Hernandez.