Fourth seed Amelie Mauresmo inflicted only the second defeat of the year on world number one Serena Williams in the Rome Masters semi-finals on Saturday.

The reward for her hard-fought 1-6, 7-5, 6-3 victory is a final showdown against second seed Kim Clijsters, who crushed Japan's Ai Sugiyama 6-3, 6-2.

Mauresmo had not beaten Williams in five previous matches. The last time they met the American triumphed 6-3, 6-2 on fast carpet in Paris earlier this year, though this was their first encounter on slow European clay.

The defending champion dominated the first set, playing perfect power tennis and rifling groundstroke winners to close out the set in just 21 minutes.

"Obviously at that point I didn't feel so good," Mauresmo told reporters.

"She was playing good tennis, she wasn't missing a thing, putting balls deep and wide. I just had to wait in there, stay in the match and hope her level came down."

The match turned in a nervy second set in which Mauresmo started positively, holding her serve to love and breaking Williams.

But then she choked, hitting two double faults at 30-30 in the following game to surrender the advantage.

In the ninth game the top seed broke to grab a 5-4 lead and a chance to serve for the match, but she also stuttered, losing her next two service games and allowing Mauresmo to level the match at one set all.

FOUGHT BACK

Mauresmo twice fought back from a break down in the decider before profiting from three wide forehands in the eighth game to snatch a 5-3 lead.

She then calmly served out the match to become only the second player this year to defeat the 21-year-old American after Belgium's Justine Henin-Hardenne achieved the feat in the final of the Charleston claycourt tournament last month.

"In that third set I realised I could stay with her. I'd already broken her a few times and I knew I could do it again," the fourth seed said.

"It was tough to come back from 2-0 down (in the third set). But mentally I just stuck in there."

She now hopes to go one better than her two runner-up finishes in Rome in 2000 and 2001.

Williams, meanwhile, attributed the loss as much to her mistakes as her opponent's skill.

"I don't think there was one single thing in particular she did (to put me under pressure)," she said.

"Every time I lose I think it's because of things I didn't do. If I'd have kept playing like I did in the first set, I would have won the match. I made far too many errors, especially on the serve."

Asked whether the defeat would affect her plans in the run-up to the French Open later this month, Williams replied: "It's always disappointing to lose, but it's better to lose here than in Paris (at the French Open). It's not the end of the world."

Clijsters completed her inexorable progress to Sunday's final with a 6-3, 6-2 win over Sugiyama.

The Japanese 13th seed led 2-0 in the first set before the 2001 Roland Garros finalist found her rhythm, striking a series of clean winners past her occasional doubles partner.

Mauresmo versus Clijsters promises to be a tight contest. They stand at two victories apiece from their previous four meetings, all on hard courts. Their two most recent matches were both settled 7-5 in the third set.

"Having watched her (Mauresmo) beat Serena today, I think with Justine (Henin-Hardenne) she's one of the best claycourt players out there at the moment, especially as she serves so well," the second seed told reporters.

"Tomorrow I'll have to be very consistent, not make any errors and be aggressive. She doesn't give you anything for free".