Flushing Meadows, NY (My Sportsbook) - Defending champion Serena Williams and 2005 champion Kim Clijsters easily won their respective quarterfinal matches on Tuesday at the U.S. Open.

The second-seeded Serena handled 10th-seeded Italian Flavia Pennetta, 6-4, 6-3, in one hour and 15 minutes under the lights at Ashe Stadium. The American fired seven aces, compared to zero for Pennetta, to secure her spot in the semifinals.

Williams, who won 61 of the 108 total points in the match, improved to 23-1 in Grand Slam singles matches this season. Williams has already won Wimbledon and the Australian Open so far this year.

The three-time U.S. Open champion Williams is in search of her 12th major title.

Pennetta failed to become the first Italian woman to reach the semifinals in a Grand Slam.

"Flavia is clearly a good player, and I was definitely not underestimating her," Williams said. "I thought it was a really good match. You know, she also has a good spirit."

Williams' victory sets up a much-anticipated battle with wild card Clijsters, who discarded 18th-seeded Chinese Li Na 6-2, 6-4 in 63 minutes at Ashe Stadium during the afternoon on Tuesday.

Clijsters converted on her second match point against Li, who misfired wide with one final forehand on Day 9 at the Billie Jean King USTA National Tennis Center. The victory marked Clijsters' 100th in Grand Slam action.

The 26-year-old Clijsters broke Li's serve four times, compared to only one break for the Chinese, who helped the Belgian's cause by piling up 41 unforced errors.

The former world No. 1 Clijsters toppled third-seeded Venus Williams here on Sunday.

"I have a really good feeling the way I stayed focused after the Venus match. I didn't lose focus just by everything that was going on around me and so much more attention that happened," Clijsters said. "I think that's something that I learned from the past, that experience that I have when you beat big players not to get carried away and just refocus on a match like today."

The Belgian star has reached at least the semis in her last six major events.

Clijsters captured the U.S. Open back in 2005 and was the runner-up here in 2003. She returned to the WTA Tour just last month after retiring in the spring of 2007 in order to start a family.

The Belgian, who has won her last 12 matches in Flushing, is playing in her first U.S. Open since winning the title here four years ago.

Serena owns an impressive 7-1 lifetime record against Clijsters, with the last meeting coming in the '03 semis at Miami.

"She's a really good player. She plays tough. She plays hard," Serena said of Clijsters. "Now it's like a totally different level, because she has absolutely nothing to lose. I think that's when you can play your ultimate best tennis."

The quarterfinals will conclude here on Wednesday when ninth-seeded Dane Caroline Wozniacki meets upstart American Melanie Oudin and Belgian teenager Yanina Wickmayer takes on Ukrainian Kateryna Bondarenko. Wozniacki was a hardcourt titlist in New Haven two weeks ago.