No. 1 seed Victoria Azarenka knocked off No. 7 seed and defending U.S. Open champion Samantha Stosur in three sets on Tuesday, 6-1, 4-6, 7-6 (7-5), to advance to the semifinals.

Azarenka had to fight for the win after Stosur shook off an uncharacteristically sluggish start to force a third set tiebreaker. But Azarenka was able to finally pull out the win in what had to be one of the toughest draws for a No. 1 seed in awhile.

Azarenka looked dominant early as Stosur struggled to find any sort of groove. But Stosur would go out like a champion in forcing the tiebreaker and putting Azarenka on her heels. Azarenka had to earn every last bit of this match.

Stosur served trailing 5-4 in the third set after Azarenka held serve to take the lead and then Stosur returned the favor by holding her own serve to make it 5-5. It went back and forth again, eventually leading to a tiebreaker that Azarenka won, 7-5. Azarenka was up 4-0 in the tiebreaker before Stosur closed it to 5-5.

Azarenka will now face the winner of No. 3-seed Maria Sharapova and No. 11-seed Marion Bartoli in the semifinals and will keep her No. 1 world ranking no matter the result of the remainder of the tournament.

A full schedule for the Round of 16 and the rest of the tournament can be found on the U.S. Open's Official Website. There, you can also find live streaming of tournament events all day long.

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