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OAKLAND, Calif. (AP) -- Frank Thomas couldn't take part in Chicago's World Series run a year ago. Now he's doing his best to prevent the White Sox from making a bid for a second straight title.

Thomas dealt another big blow to his former team's dwindling playoff hopes, hitting a three-run homer Sunday to help the Oakland Athletics beat Chicago 5-4.

"He had good at-bats and lifted their team a couple of times," White Sox manager Ozzie Guillen said. "He's a guy when he's swinging the bat he's dangerous. He's one of the best hitters I've ever seen. I think the big difference between winning and losing was Frank."

Thomas homered twice and drove in seven runs in the final two games of Oakland's three-game sweep. Chicago fell four games behind Minnesota in the wild-card race and remained five games behind AL Central-leading Detroit.

With only two weeks remaining, time is running short for the White Sox. They open a seven-game homestand Monday against Detroit and end the season with three games in Minnesota.

"We still get to play the teams in our division," outfielder Jermaine Dye said. "As long as you have that, you still have a chance. We have to come out at our place and win at least five games and hopefully we can do that. Then we need to go on the road and do what we're capable of doing."

Thomas had a bitter breakup after spending the first 16 seasons of his stellar career in Chicago, feuding with general manager Ken Williams after the White Sox decided to cut ties with the Big Hurt.

Thomas turned those hurt feelings into a golden opportunity in Oakland, where he is leading the A's on their playoff push. Oakland moved seven games ahead of Los Angeles in the AL West after the Angels lost 8-1 at Texas.

Oakland struggled the past two Septembers, losing out on the division race to the Angels both times. But with Thomas leading the charge this season, the A's are in great shape to return to the playoffs for the first time since 2003.

Thomas is batting .344 with a major league-leading 10 homers and 26 RBI this month. He has 38 homers and 105 RBI on the season, after missing large chunks of the past two years with injuries.

"After the start I had, it feels good just to be sitting here at the end of the season," said Thomas, who was batting .178 through his first 34 games. "I found my swing and found my game."

Chicago broke out to a 2-0 lead against Joe Blanton (16-11) before Thomas erased it with one swing in the fourth inning. With runners on second and third and one out and the struggling Eric Chavez on deck, Guillen decided to pitch to Thomas.

Thomas fell behind 0-2 before working the count full against Jose Contreras (13-8). He then connected for his 486th career homer, a three-run drive that gave Oakland the lead. Thomas is 7-for-19 with four homers and nine RBI in six games against Chicago.

"When he gets in a situation when there's a big hit needed more than likely he's going to come through," A's center fielder Mark Kotsay said. "The confidence is there. He's swinging the bat well. He's stepped up for us all season."

Milton Bradley added an RBI single in the fifth and Mark Ellis hit a sacrifice fly in the sixth for Oakland. Blanton allowed two runs and eight hits in five innings.

Chicago scored twice in the seventh against Brad Halsey on a wild pitch and an RBI groundout by Jim Thome to cut it to 5-4. Justin Duchscherer pitched a perfect ninth for his ninth save in 11 chances.

The White Sox had their chances but failed to score after loading the bases with one out in the first and fifth innings. In the first, Joe Crede hit a fly to Kotsay, who threw a one-hop strike to Jason Kendall to nail Ross Gload at the plate for the double play.

In the fifth, Blanton struck out Crede and retired A.J. Pierzynski on an infield popup to preserve the 3-2 lead.

"We had our chances," Dye said. "We had the bases loaded a couple of times and we just couldn't come up with that one hit."

Chicago scored twice in the second on Pierzynski's leadoff homer Scott Podsednik's two-out RBI single.

Game notes

Contreras allowed five runs and eight hits in six innings. ... Oakland is 23-4 at home against the White Sox since 2001. ... Chicago's Nos. 3, 4, and 5 hitters went 4-for-32 in the series.