France's Marion Bartoli ended world number one Victoria Azarenka's streak of 26 straight wins with an impressive 6-3 6-3 victory at the Sony Ericsson Open on Wednesday.

Seventh-seed Bartoli moves into the semi-finals where she will face Poland's Agnieszka Radwanska, a 6-4 6-1 victor over Venus Williams.

Bartoli played with aggression and confidence from the outset to take the game to Azarenka, who appeared short of inspiration and heavy on her feet.

The Frenchwoman produced a superb cross court return to break and go 4-3 up in the second set - the decisive blow which finished off the Belarussian's resistance.

"I just went out on the court full of belief in myself," said a delighted Bartoli, who raised her arms aloft in delight at beating the top seed.

Williams, playing in her first singles tournament since her shock withdrawal from last September's U.S. Open with Sjogren's Syndrome, said her late-night match against Ana Ivanovic on Monday had taken its toll.

"I was up until like 4am recovering from the match and all the things that you have to do after," said Williams. "So the next day I think I should have rested more."

The auto-immune disease causes fatigue and joint soreness and, after doing well to reach the last eight by outlasting Ivanovic in three sets, Williams looked sluggish in the latter stages of Wednesday's 80-minute match.

World number four Radwanska, who won in Dubai in February, enjoyed a 74 percent first serve percentage while Williams, who entered the tournament as a wildcard, paid the price for 33 unforced errors.

Williams, however, was encouraged to have made it to the last eight in her first tournament back.

"This is definitely a start," said Williams, whose sister Serena was eliminated byCaroline Wozniacki on Tuesday, "I've definitely learned a lot about ways that I could prepare during the tournament."