ROME (Reuters) - Venus Williams knocked defending champion Martina Hingis out of the Italian Open in three thrilling sets Saturday to reach her second successive final at the Foro Italico.

The third-seeded American took one hour 28 minutes to win 6-4 1-6 6-4 and avenge both her own defeat by the world number one in last year's final in Rome and her younger sister Serena's defeat in the quarter-finals on Friday.

``My first three matches here I wasn't playing well. I knew I had to raise the level of my game or suffer the consequences,'' said Williams.

Hingis felt the roots of only her fifth defeat of the year -- and her first on clay to Venus -- lay in the ease with which she had eliminated the other half of the Williams family.

``I underestimated Venus. I was too confident going into the match after yesterday's win over Serena and I was expecting the same thing. But she served better than Serena,'' said Hingis.

Using her lethal armory of groundstrokes and the occasional drop shot, Williams pounded Hingis into submission, wrapping up the match by breaking her opponent's serve to love.

``I guess I was just out of oxygen in the final points,'' admitted Hingis who had won both her previous matches on clay against Venus -- at Rome and Roland Garros last year.

``All through the match I was three meters behind the baseline and you can't do that against Venus.''

Even though Hingis counter-punched brilliantly after losing the first set to take eight of the next nine games, leveling the match and leading 2-0 with a break in the third, she never looked in control of her opponent or the match.

Instead, in the final set, Williams rid her game of the unforced errors that had peppered the second set surrender.

``I just decided not to miss any more. I was just missing too much in the second set -- and it just wasn't to my advantage,'' said Williams.

The American, who has already won three tournaments this year, will meet French fourth seed Mary Pierce in Sunday's final in a repeat of last weekend's Hamburg Open.

Williams won that match, gaining her first clay-court title when Pierce retired hurt in the second set. This time the French player will need a fitness test on a strained thigh before knowing whether she will even start the final.

Pierce made light of the injury while defeating compatriot Amelie Mauresmo 6-7 6-2 6-4 in the day's first semifinal.

Asked about it, she said: ``I'll have to find out what the trainer says. When you're out there playing you can just keep going because of the adrenaline and warmth.

``I hope I'm not going to cool down too much because at the moment it's hurting a little.''

The match turned at the start of the second set on the fourth seed's abrupt change of strategy.

Instead of relying on powerful groundstrokes, Pierce started lobbing deep to the baseline, pushing Mauresmo out of court and out of the tournament.