Defending champion Serena Williams slugged her way to a US Open title rematch against Victoria Azarenka as the top two seeds scored straight-sets knockouts in their semifinals on Friday.

World number one Williams thrashed a rattled Li Na 6-0, 6-3 overcoming some staunch resistance at the end, while Azarenka returned to the Flushing Meadows final with a 6-4, 6-2 win over unseeded Italian Flavia Pennetta.

"Just to be able to defend a title for once would be really awesome," said French Open winner Williams, who will get her chance on Sunday after failing to go back-to-back following her 1999, 2002 and 2008 titles in New York.

Although overpowered in the opening set, Li battled valiantly in the second, saving six match points to hold serve before the top seed finally closed the deal on a seventh with an unreturned serve.

Williams, hot off a 6-0, 6-0 quarterfinal win against Spain's Carla Suarez Navarro, had won 24 consecutive games before Li held serve in the second game of the second set.
"To hear constantly, 'go Serena, go Serena,' it is really a pleasure. I hear young voices and older voices and it really makes me feel so special," said Williams. Williams and Azarenka's final will be the first back-to-back title clash at the US Open since Serena and her older sister Venus battled for the crown in 2002.

Belarusian Azarenka trails 12-3 head-to-head against Williams, but has won two of their last three matches including the two hardcourt finals in Doha and Cincinnati.
"When you play against Serena, you have to play your best," said Azarenka.

Williams is bidding for a fifth US Open title and her 17th in Grand Slams.
At 31, she would become the oldest women's winner at Flushing Meadows since tennis turned professional in 1968, supplanting Australia's Margaret Court who won in 1973.
Australian Open champion Azarenka booked her return to the Championship match by finally finding a way to hold serve in a contest littered with 13 service breaks. In the end, Azarenka's power was too much for the battling Pennetta, who returned from a serious wrist injury to reach her first Grand Slam semi-final.

There were five service breaks in a row before Azarenka held to win the first set after her unseeded 31-year-old opponent saved five set points. The second set followed a similar pattern, with three broken serves before Azarenka turned up the power.

After holding serve to lead 3-1, Azarenka broke Pennetta yet again and squealed with delight after closing out the 94-minute match with a well-placed forehand.