Azarenka beat Italy’s FlaviaPennetta 6 -4, 6-2 in a match with 13 breaks of serve.
World number one Serena Williams needed seven match points to seal a convincing 6-0 6-3 win in the second semi-final.
The American has only dropped 16 games in the tournament.
“It was a good match,” said Williams. “Li Na is such a great player. I got a little nervous but I was able to close it out, finally.”
Li Na was expected to provide some sort of competition for the four-time champion and was actually leading in number of aces chart heading to this game but Serena proved to be too strong for her.
An early double fault unsettled Li Na, who lost the first set in 29 minutes with Serena Williams putting her pressure by returning 84% of her serves.
She finally got on the scoreboard after almost 40 minutes and broke Williams, the fans cheered her on but the rally was short-lived.
It was a minor blip for Williams, however, as she took the next four games in a row as she moved one game from victory.
The final 15 minutes were the best of the contest with Li Na fighting to stay in the game saving five match points.
Williams was nervous, despite her commanding lead but converted her seventh chance with a first serve to reach her seventh Us Open final.
“Today it doesn’t matter who is the opponent,” said Li. “The problem is myself. Even if today I was playing against maybe the player ranked 100, still the same problem.”
Azarenka was a strong favourite in the first semi-final against a player making her first Grand Slam semi-final at the age of 31 and she proved to be no pushover.
Seven out of nine games went against serve as an off-key Azarenka leaked errors, while Penetta fought hard to defend her second serve.
Azarenka managed to take the opening set by holding on to her serve in a tight tenth game. With the first game secured the Belarusian began to play her game and took a 3-1 lead with a succession of magnificent forehands, rounding off with a thumping drive volley.
Pennetta cut the deficit with a another break to trail 4-2 but despite a couple of flashing returns she could not prevent the world number two from serving out the match.
“I’m happy I’m in the final,” said Azarenka. “There are always things that can be better. I felt like the first set was a little bit tricky, the momentum wasn’t there a little bit.
“Just one shot wasn’t working and threw my rhythm a little bit off balance.”
The final is expected to be played today.