Venus and Serena Williams made it through tough three-setters in the quarter-finals on Friday.
The WTA Championships bring together the top eight players in the world and begin with round-robin play Tuesday in Istanbul.
Top-seeded Novak Djokovic and Serena Williams overcame tough challenges Thursday to reach the quarterfinals of the China Open.
Her 17 Grand Slam titles are the sixth-most in history for a woman, only one behind Martina Navratilova and Chris Evert.
Roger Federer, Rafael Nadal, Andy Murray and Novak Djokovic have won 33 of the last 34 majors dating to 2005.
Serena Williams will be seeded No. 1 at the U.S. Open as she tries to do something she's never done: win a second consecutive title at Flushing Meadows.
No. 2-seeded Roger Federer was broken in the opening game but held the rest of the way and eased into the fourth round.
Nadal improved to 20-10 in his career against Federer, and showed once again that he'll be the player to beat when the French Open starts next Sunday.
Venus Williams lost to Laura Robson 6-3, 6-2 in the wind at the Italian Open on Monday, ending prospects for a sister showdown in the second round.
Top-ranked Serena Williams outlasted Anabel Medina Garrigues 6-3, 0-6, 7-5 to reach the Madrid Open semifinals on Friday.
