By Nick Crowther
BBC Sport at Wimbledon
Serena Williams stayed on course for a third successive Wimbledon title but was made to battle by Amelie Mauresmo.
Defending champion Williams was stretched by the fourth seed
The Frenchwoman started poorly, making unforced errors and being broken in the first game, before recovering to force a tie-break, which she won 7-4.
Mauresmo was in charge early in the second, but Williams upped her game as the fourth seed struggled with a back injury and edged a tight set.
Williams dug deep to win the third for a 6-7 (4-7) 7-5 6-4 victory.
"This is definitely the most special moment in my career," the 22-year-old told BBC Sport after the thrilling clash.
"I'm really excited to be in the final because it's been a tough year."
The American is bidding to become the first player since Steffi Graf to win the women's singles title in SW19 three times in a row.
She had beaten Mauresmo at the same stage in 2002 en route to her first title, but the Frenchwoman denied it was the mental side that let her down.
"Probably the fact that I couldn't serve well in the third set made it a little easier for her but I don't think its mental," she said.
"In these matches it's just a couple of points here or there. I was a break up in the second set and couldn't hold my serve."
Williams was on course when Mauresmo double-faulted twice and sent two volleys into the net in the opening game before seizing the first opportunity to break with a forehand winner.
But Mauresmo was inspired and though Williams twice saved break points with an ace, Mauresmo broke back to force a tie-break and won it 7-4.
The fourth seed was in command at that stage and a backhand winner saw her break Williams to go 3-1 up.
Everything seemed to be going against the top seed - even a bad bounce in the next game.
But some of the notes she was reading during the changeovers seemed to work and she upped her performance to break back as her opponent needed treatment on a back problem.
Three more breaks of serve followed but Mauresmo double-faulted to lose her serve and the set 7-5.
Williams had the first chance at 2-1 in the decider but Mauresmo, moving a little more freely, saved it with a volley.
Another series of superb rallies followed but Serena won with a single break when Mauresmo sent a forehand wide serving at 4-5 down.
"I think it was the toughest match we've had, " said Williams. "I just tried to fight for every point.
"It feels like I'm playing good tennis. I'm finally back."
And Mauresmo insisted the disappointment was not as acute as following her defeat to Elena Dementieva at the French Open last month.
"There's no comparison between the two to me," she said.
"In Paris I didn't play my game or give 100%, I was too tense. I feel I come off the court here having played some great tennis.
"I've improved a lot and I'm going to get there."