Josh Reddick spent some time during batting practice Friday needling A's teammate Josh Donaldson.

"I told him he wasn't strong enough to go the other way," Reddick said.

Reddick has seldom been happier to have been proved wrong. Donaldson delivered a monstrous and timely opposite-field home run with the bases loaded in the sixth inning, giving the A's all the runs they would get off Chicago White Sox ace lefty Chris Sale in a 4-3 win.

That victory, coupled with Texas' 6-1 loss in Toronto, moved the A's into first place in the American League West by one-half game.

"JD has put us on his back," Reddick said, looking at Donaldson's team-best .329 average and 41 RBIs. "At some points he's carried us."

The A's have won 18 of their last 21 games, including beating Sale twice. This is a left-hander who won 17 games last year and had a 28-inning scoreless streak before Donaldson's sacrifice fly against him Sunday in Oakland carried the A's to a 2-0 win.

In his last 361/3 innings against all comers, Sale has allowed five runs. Donaldson has driven in all five.

"He's one of the best left-handers in the league, if not the best," Donaldson said. He picked on a 1-1 fastball up and away and hit it the other way, just as Reddick suggested he couldn't. "I wasn't trying to do too much.

"I'd already had two at-bats against him and was feeling very comfortable. He had to come in to me at that time, and I feel that any time a guy has to come at me, I'm going to be aggressive."

Being aggressive is second nature for Donaldson. He's now 10 for 18 in his career when batting with the bases loaded. He wasn't necessarily supposed to be the A's cleanup hitter against lefty pitching, but it's going to be difficult to get him out of that role. He's carrying a .350 average against left-handed pitchers, and five of his nine homers have come off lefties.

"That was big-time stuff right there," manager Bob Melvin said. "He's got a consistent approach and he understands what he has to do."

Before the Donaldson slam, the first of the year for the A's, the White Sox were able to gingerly build a 3-0 lead. The first of three hits for Tyler Flowers, a homer, was the first hit off Jarrod Parker in the third inning.