American Serena Williams and Russia's Maria Sharapova continued their impressive run of form at the WTA Tour Championships in Istanbul, winning their final matches of the group phase in straight sets.

Williams won her third match of the year-end championships Thursday by defeating world No. 1 Victoria Azarenka of Belarus, who brought a 14-match winning streak into the much-anticipated Red Group contest.

The showdown was also a rematch of this year's U.S. Open final, which Williams rallied to win in three sets.

Azarenka had a chance to break serve in the first set when Williams fell behind 0-40, but the American's powerful serve bailed her out of trouble and she stayed in the lead at 3-2.

The players stayed on serve until 3-4, when Azarenka dropped her service with a double fault. She broke right back but then surrendered her serve again in the 10th game to drop the set.

Azarenka started off more aggressively in the second set and improved her first-serve percentage en route to grabbing a 3-0 lead, but Williams's power proved too much and she rallied to take the match 6-4, 6-4.

With the win, Williams continued her total domination of the head-to-head rivalry, which she now leads 11-1.

In White Group action, Sharapova remained undefeated by handily defeating Australia's Sam Stosur on Friday 6-0, 6-3. Stosur had taken the place of defending champion Czech Petra Kvitova, who withdrew from the event after her first match loss due to a viral illness.

Italy's Sara Errani and Poland's Agnieszka Radwanska were squaring off Friday in another White Group match, with the winner to advance to the semifinals to take on Williams.

The late match Friday will pit Azarenka against China's Na Li for a spot in the semifinals and a matchup against Sharapova.

Azarenka also can lock up the year-end No. 1 ranking for the first time by winning that match, while Sharapova, the world No. 2, still has an outside chance of snagging the top spot if the Belarusian falls short. EFE