* Williams routs China's Li in straight sets
* Azarenka cruises past Italian Pennetta
* Williams sisters eliminated in women's doubles (Adds quotes, details)
By Larry Fine
NEW YORK, Sept 6 (Reuters) - Defending champion Serena Williams slugged her way to a U.S. Open title rematch against Victoria Azarenka as the top two seeds scored straight-sets knockouts in their semi-finals 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.
"It was a good match. It was tough at the end," Williams said. "I got a little nervous but I was able to close it out."
Former French Open champion Li, China's first semi-finalist at the U.S. Open, admitted that she experienced a bout of nerves when she hit the Arthur Ashe Stadium court.
"I should not be, like, nervous because (it was) not (the) first time to play semis," said Li, twice an Australian Open finalist. "But when I walked to the court I was feeling the court (was) so big. I mean, even my side, it was feeling like a football court.
"In the end, finally, I can play tennis," she added with a smile.
Williams, hot off a 6-0 6-0 quarter-final 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 U.S. 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, hardcourt finals in Doha and Cincinnati.
"When you play against Serena, you have to play your best," said Azarenka. "She makes me play my best."
Williams is bidding for a fifth U.S. 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.
"It's an amazing opportunity, something that every tennis player dreams of, to be in the final of the U.S. Open and play against the best player in the world," said the second seed, a former U.S. Open junior champion and mixed doubles winner.
"I can't wait to go out and play."
Earlier, Belarusian veteran Max Mirnyi captured his third U.S. Open mixed doubles title on Friday when he teamed up with Czech Andrea Hlavackova to beat American Abigail Spears and Mexican Santiago Gonzalez 7-6 (5) 6-3.
The 36-year-old Mirnyi, the reigning Olympic mixed doubles champion with compatriot Azarenka, won his first U.S. Open mixed doubles title with Serena Williams in 1998 and teamed up with Azarenka for his second in 2007.
Hlavackova later returned to the court with fellow Czech Lucie Hradecka and beat Serena and Venus Williams 6-4 6-2 in their women's doubles semi-final, which came less than an hour after Serena's singles semi-final.
The Czechs will play the Australian duo of Ashleigh Barty and Casey Dellacqua in the final that follows the men's singles semi-finals in Arthur Ashe Stadium on Saturday.
Top seed Novak Djokovic opens the day against ninth-seeded Swiss Stanislas Wawrinka, with second seed Rafa Nadal taking the court next against eighth seed Richard Gasquet of France.
(Editing by Frank Pingue/Ian Ransom)