She next faces unseeded Virginie Razzano of France, who put out 16th seed Kaia Kanepi of Estonia 6-1 6-2.

World number one Serena double-faulted twice as Venus broke for a 1-0 lead and then twice more when she broke again to go 4-1 up in the first set.

While Serena struggled to get going, Venus swept through her three service games without conceding a point.

Serena struck the ball much better in the second set, forcing errors instead of making them, and broke serve in the third and seventh games to level the match.

In a highly competitive final set, Serena broke to lead 2-1. She then saved three break points with aces in a remarkable sixth game before conceding a fourth break point.

World number six Venus then had to serve her way out of trouble at 5-5, firing two aces on break points before going on to take the tiebreak 7-3.

"It was definitely a finals-quality match," said Venus, who is back in the top five for the first time since August 2003.

BEST PART

"Obviously, winning a match that close is satisfying, especially against the top-ranked player. Of course, when I walk on to the court with Serena I have respect for her, so that's obviously the best part about winning."

Serena thought their serving had made the difference.

"I didn't serve that well and I thought Venus served really well which I think ultimately helped her win the match," she told reporters.

Serena appeared to be having problems with her left leg and put it down to playing so many matches this year.

"I'm just playing every match as hard as I can whether I'm hurt or not," she said. "I've just been playing a lot of matches, more than I have in my career, and I'm a little tired."

Razzano continued a remarkable run this week that has included upset victories over second seed Dinara Safina and number five Vera Zvonareva.

Kanepi, who beat world number three Jelena Jankovic on her way to the semis, broke serve in the opening game but looked weary for most of the match.

Razzano struck the ball more positively and was more mobile than her opponent. After breaking back for 1-1 she reeled off the next seven games to take the first set and lead 2-0 in the second.

The Frenchwoman had a break point to lead 3-0 in the second but netted her return and Kanepi went on to hold with an ace.

Kanepi then broke for 2-2 when her opponent double-faulted before netting a weak forehand.

Razzano, however, quickly regained control and claimed the final four games for the loss of three points to wrap up the match in 63 minutes.

"It was me who was the boss today," said Razzano. "I wanted to do my best to reach the final."