Williams was trying to win her third title of 2007

Switzerland's Schnyder held her nerve in a final set tie-break to come through 6-3 2-6 7-6 (7-5) and reach the semi-finals.

Before Rome, Williams had won two of the three events she had completed this year in Australia and Miami.

Schnyder will next face third seed Jelena Jankovic after the Serbian thrashed Elena Dementieva 6-2 6-1.

Williams, the Australian Open champion, was left to reflect on a wayward performance.

"I just didn't make the shots that I should have. I probably hit about 50 unforced errors, which is pretty good to get to 7-6 in the third set," she said.

I'm just really surprised that I won Patty Schnyder

"I think not winning today is going to work well for me because it will give me time to get even fitter for the French Open. As soon as I start to believe in myself I'm going to be a lot better."

But Schnyder admitted that luck was on her side against Williams.

"I'm just really surprised that I won. In the first set she was missing a lot, then in the second my game went wrong," she said.

"The third set was just really tough, very close. I tried to mix it up, but she's such a champion that you have to expect her to hit a lot of winners. I'm really happy and lucky to be through to the next round."

In the other half of the draw, second seed Svetlana Kuznetsova also progressed with a 6-1 6-3 win over fellow Russian Dinara Safina.

She will now play ninth seed Daniela Hantuchova, who defeated Spain's Anabel Medina Garrigues 7-6 7-5.