Williams, ranked No. 9 in the world, and seeded fifth this week, is looking for her third tournament title of the year.
She will face ninth-seeded Vera Zvonareva in today's final of the $1.3 million tournament. Zvonareva defeated Elena Dementieva 6-3, 6-7 (2), 6-3.
Williams trailed 4-1 in the first set, but fought back, breaking Cornet twice at love before taking the set. In a second set featuring long baseline rallies, Williams broke Cornet in the eighth game, then held serve at love to take the match.
"In just the whole match I thought she played really well, and I just wasn't playing my game," said Williams who with the win guaranteed she will surpass the $19 million mark in career earnings. "I got caught a little bit on the defensive a lot and I wasn't doing anything I should have been doing."
Cornet, an 18-year-old ranked No. 40 in the world, also made the semifinals at Amelia Island last week. She was runner-up earlier this year at Acapulco, her best showing on the tour which she joined last year.
Saturday was the first time she faced Williams.
"It was a little bit of a surprise for her because she didn't know me," Cornet said. "I was like making the pressure on her and it was working and after 4-1, you know, she played better."
Cornet said she has learned a lesson by reaching two consecutive semifinals.
"I have to fight and I have to play my game, and I think I have the level now for beating the best, so it's a lot of confidence for me," she said.
Williams said she will surely be facing Cornet again.
"She gets a lot of balls back, and those kind of players are usually around for a good 20 years," Williams said.