Schiavone, seeded sixth, was the underdog going into the match but was full of energy and regularly found her best shots just as Williams looked to be taking control.

There were two breaks apiece in the first set, leading to a thrilling tie-break that saw French Open champion Schiavone at her most vocal as she berated herself for the missed opportunities that allowed Williams to go ahead.

Two-time champion Williams looked she might make life easier in the second set when she broke Schiavone in the fourth game then served out the next to go 4-1 ahead.

But Schiavone, backed into a corner, came out fighting and took the next two games.

It had looked as though she had missed her chance to break back when she failed to convert a pair of chances at the net with the score at 30-40, but she held her nerve to put the pressure back on Williams.

In a match characterised by strong return games and loose serving Williams immediately added another break of her own meaning at 5-3 she had the chance to serve for a a place in the last four.

What followed was a tame attempt which culminated in a big double fault.

With the possibility of another tie-break looming, Schiavone dropped serve for a fifth time to see Williams into a semi-final against either Sam Stosur or number two seed Kim Clijsters.