It was one way traffic early on in the match, as the No.1-seeded Williams came out firing on all cylinders, winning the first seven games and building a 60 10 lead. The fantastic start extended her run to 24 games in a row - she had won the last five games of her fourth round match against No.15 seed Sloane Stephens and all 12 games of her quarterfinal against No.18 seed Carla Suárez Navarro.
The No.5-seeded Li picked her moment and pounced, holding for 1-all then breaking for 2-1, and even getting to 40-0 in that game. The momentum was clearly flipping - but Williams would have none of it, digging out of that game to get the break back and then reeling off the next three to go ahead 5-2.
Then came the best game of the match. A resilient Li fought off six match points in that game - many of them with winners - and, after almost 14 minutes, finally held serve again to close the gap to 5-3.
But the resistance was short-lived, as Williams confidently served it out in the next game, blasting a big serve up the middle that drew a forehand error from Li. Game, set and match to Williams, 60 63.
"Li Na's such a great player," Williams said in her on-court interview with Mary Joe Fernandez. "It was tough at the end of the match, and I got a little bit nervous there, but I was able to close it out finally.
"I've just been really focused in all of my practices and I've been trying really hard, so to come out here and constantly hear, 'Go Serena, Go Serena,' it's honestly a pleasure. I hear young voices and older voices out here and it really feels so special, so thank you very much to the fans and everyone."
Williams was asked in her post-match press conference about the thought of Grand Slam No.17.
"I thought about that, but I can't think about that," Williams said. "It's still so close but it's still so far. I have to win against a great player and I have to play great tennis. With that being said, I have to do it all over again. Nothing is guaranteed. Only thing that's guaranteed is I can go out there and just try to do my best, and whatever happens, just learn from whatever experience I get and move forward."
And what about the potential $1 million bonus for winning the Emirates Airline US Open Series?
"Well, last time I had this opportunity, it blew up. It didn't work out. I'm not trying to think about it so much. Money has never been my motivation with tennis. I've always played to win Grand Slams."
Awaiting Williams in the final is No.2 seed Victoria Azarenka, who has actually won two of their three meetings this year, in the finals of Doha and Cincinnati. The World No.1's thoughts on her?
"Both matches she played really well," Williams said. "I let myself down at opportunities and I have to take them, because she is always pumped up and she does really well. I definitely feel like when she plays me she plays her best, by far. I've seen her play against other players, and when I play her I'm playing a totally different player. Obviously she brings her best game when she plays against me."
Li was in her first US Open semifinal, having reached the semifinals or better at the Australian Open and French Open - the only one missing is Wimbledon, where she is a three-time quarterfinalist, in 2006 (falling to Kim Clijsters), 2009 (falling to Williams) and 2013 (losing to Agnieszka Radwanska).