Serena strolls into Stanford semis

However, the Swiss veteran's struggles against Williams - who has now won eight of their 11 contests - continued.

"It was definitely quick," Williams said of the 54-minute victory. "With a player like Patty, you have to be quicker because she is such a big shot-maker you never know what is going to happen."

Williams next faces Candian Aleksandra Wozniak, who beat Australian Samantha Stosur 6-2, 5-7, 6-4.

Wozniak had beaten Stosur in straight sets in the final round of qualifying on Monday, but Stosur was awarded a place in the main draw as a lucky loser when American Lindsay Davenport withdrew with a knee injury.

Wozniak, 20, made the most of her first quarter-final of the season, and now will vie to reach her first WTA final since last season at Fes.

In other quarter-final matches, Japanese veteran Ai Sugiyama advanced after 19-year-old Slovakian Dominika Cibulkova was forced to retire with a left leg cramp.