Flavia Pennetta of Italy Proved a Weak Opponent Against Mighty Serena Williams

Defending champion Serena Williams has advanced to the U.S. Open semifinals, setting up a showdown against Kim Clijsters.

The No. 2-seeded Williams beat No. 10 Flavia Pennetta of Italy 6-4, 6-3 Tuesday night to improve to 23-1 in Grand Slam singles matches this season.

Williams has won three of the past four major championships. She is seeking her fourth U.S. Open title and 12th Grand Slam overall.

With her older sister Venus sitting in the stands, Williams piled up a 21-9 edge in winners against Pennetta, who was trying to be the first Italian woman to reach a Grand Slam semifinal.

Clijsters, the 2005 U.S. Open champion, beat No. 18 Li Na in straight sets earlier Tuesday.