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The American fought back from a set and 3-1 down to win 4-6 7-6 (7-2) 6-4 in front of her home Los Angeles crowd.

Mauresmo needed to win the tournament to overtake Lindsay Davenport for the number one ranking.

Wimbledon champion Maria Sharapova beat fellow Russian Anastasia Myskina 2-6 6-2 6-2 in the other semi-final.

Mauresmo quickly went two breaks up against Williams, but faltered before serving out the set.

Williams looked in trouble when she dropped serve immediately in the second set but took advantage of a string of Mauresmo mistakes by breaking back.

The American raced through the tie-break before claiming the key break at 3-3 in the decider.

I'm going to be a force to be reckoned with next year Serena Williams

Mauresmo said: "I was playing some good tennis and she really raised her level a lot.

"I felt like I forced her to play her best level and really make some passing shots and make some unbelievable saves."

Williams, who has won only two titles all season, was delighted with her form.

"I'm doing in the match what do I in practice, so for me that's really important," she said.

"I'm really excited about myself. I'm going to be a force to be reckoned with next year.

"And I'm really happy for Lindsay. She's had a few tough years.

I'm so excited to be in the final, I don't care about my chances Maria Sharapova

"To come back as number one kind of motivates me because I know she had a big knee injury as well."

Meanwhile, French Open champion Myskina said Sharapova would have to raise her game if she wanted to repeat her Wimbledon triumph.

"If Serena plays like she did today, I don't think Sharapova has any chance," said Myskina

"I got real tired and lost my concentration."

Sharapova countered: "I'm so excited to be in the final, I don't care about my chances.