They're looking for Jered Weaver to continue tilting the race in their favor.
Lester and Weaver will start opposite each other for the first time Friday night, when the A's and Angels continue this key four-game set in Anaheim.
Oakland (78-55) acquired Lester (13-8, 2.53 ERA) from Boston last month in the hopes of fortifying its chances at a West title, and he's certainly done his part by going 3-1 with a 2.60 ERA in five starts.
"No matter what the expectations are that people put on me, they are not going to be as high as the expectations I have for myself," Lester told MLB's official website. "I expect myself every time out to throw nine shutout innings, whether it's April or October."
The A's, though, have dropped 11 of 17 games and are now expecting Lester to cut into the two-game gap on first-place Los Angeles (80-53) after dropping Thursday's opener 4-3 in 10 innings.
This will be the left-hander's second start against the Angels since the trade, after he held them to one run with seven strikeouts in seven innings while not getting a decision in a 2-1 home win Saturday.
"He brings total tenacity," manager Bob Melvin told MLB's official website. "He keeps guys on their toes. He works quick. You can feel the intensity behind him. When you have a big game, he's the guy you want on the mound."
The Angels, winners of 12 of 16, may feel the same about Weaver (14-7, 3.72), who leads the team in wins and is also 7-3 with a 2.58 ERA over his past 13 home starts.
The right-hander, however, is 1-2 with a 6.00 ERA in three starts against the A's this year, and was charged with a 7-1 loss in the only one at home on June 11. He gave up four runs in 5 1-3 innings after beating Oakland in four straight games in Anaheim while yielding one run in 27 1-3.
Like Lester, Weaver made his presence felt in the division race last weekend. He struck out eight and permitted three runs and two homers in 6 2-3 innings of Sunday's 9-4 victory, preventing the Angels from being swept in that three-game series.
Howie Kendrick made sure Los Angeles wouldn't be in the same situation this time, as he connected for a walkoff sacrifice fly Thursday. The second baseman is hitting .324 (12 for 37) over the past nine games, including 5 for 11 in the past three meetings with Oakland.
"Every time we play these guys, it's a great game," said Kendrick, who had two hits in the series opener. "We've already had a few walk-offs against these guys. We know we've got to play them ... down the stretch."
Kendrick, though, is 2 for 12 against Lester since 2010, and Mike Trout hasn't done any better by going 0 for 6 with three strikeouts. However, Trout is hitting .341 with four homers and nine RBIs over his last 10 games against the A's.
Oakland's Josh Donaldson is 7 for 13 with a homer and four RBIs in four games since a knee injury sidelined him for Sunday's meeting with the Angels. His production may not have been missed, however, since he's 1 for 11 against Weaver.