Williams beat the fifth-seeded Li, 6-0, 6-3, to advance to the U.S. Open finals.
There was a nearly 14-minute game when Li saved six match points in the second set, and it was full of creative points worthy of the standing ovations both players received. But when Li finally won the game it was only to keep her behind, 5-3, with Williams ready to serve out the match.
Which she did, hitting a lob and a service winner for her last two points.
"I got a little nervous," said Williams, who will face No. 2 Victoria Azarenka of Belarus on Sunday in a rematch of last year's final, "but I was able to close it out, finally."
Williams, 31, who is aiming for her 17th Grand Slam title and fifth U.S. Open championship, said, "I think I started out really well. I was really focused. Li Na may have been a little bit off. but that's because I started out so well.
"Towards the end I think it was a quality match."
Williams hasn't lost a set in her six matches here and no more than four games in a single match (against Sloane Stephens.).
"In the end, I finally came to play tennis," said Li, also 31. "In the first set, it wasn't about the technique. It was about myself. I cannot face two problems. First time in the semis, I think about the situation. I was feeling I need more practice.
"Maybe, I wish, in another six months I can face her again."
Williams had only four aces and her average service speed was 105 mph, only four mph faster than Li. Each player won 63% of first-serve points, a statistic Williams normally dominates.
There were also 20 unforced errors for Williams, an unusually high number, compared to 19 winners. But Li had only eight winners.
In the first semifinal, 24-year-old Azarenka, the only under-30 player in the women's semis, made unseeded Flavia Pennetta of Italy run as if she were chasing a rabbit. And when Pennetta would get close to that rabbit, uh, tennis ball, she often had no play.
The tennis was sometimes sloppy — the two combined for 48 unforced errors — but it was second-seeded Azarenka who advanced to the finals for the second straight year with a 6-4, 6-2 win. This was the 31-year-old Pennetta's firstGrand Slam semifinal.
Pennetta, who missed this event last year after having wrist surgery, said, "At the end, she's a powerful player. She had more power than me. Today I lost to the No. 2 in the world. I tried my best, but it's been a really good week for me. I have three more tournaments. The season is not finished yet."
Pennetta came here ranked 83rd in the world and will leave ranked 31st when the new WTA points are posted Monday.
Azarenka was in a good mood, dancing to the Pointer Sisters to warm up and listening to Marvin Gaye to cool down.
Pennetta lost her serve eight of nine times and Azarenka had four double faults in her first two service games. "Besides the breaks," she said, "I felt I was very solid from the ground today. There is just one piece missing from my serve to put all the game together.
The day wasn't a total success for Williams, who lost with sister Venus in the women's doubles semifinals to Andrea Hlavackova and Lucie Hradecka of the Czech Republic.
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