"I think it was raising from the first point, the tension, the battle, the determination," the second seed said about the quality of the match. "It was really kind of like boiling the water. It felt from every point, it was rising the level."
The Belarusian showed her fight and took advantage of a string of Williams errors to break right back for 4-2 and rode that momentum.
Twice Williams served for the match, at 5-4 and again at 6-5, but Azarenka rose up to break the American's serve and force a tiebreaker.
Williams took a 3-1 lead but Azarenka won five of the next six points to seize a 6-4 advantage and sent the match to a third set when Williams belted a backhand long to lose it 8-6.
"I think I got a little uptight," Williams said about squandering her chances. "I wasn't playing smart tennis. (But) you have to keep fighting for everything."
The third set stayed on serve until the fourth game when another double fault, her seventh of the match, sank Azarenka and handed Williams a 3-1 lead.
With the stadium crowd roaring their support for the home favorite, the American broke Azarenka two games later for good measure to make it 5-1 before she claimed victory when the Belarusian sent a backhand long on the second match point.
Williams blasted 36 winners in the match against 17 for Azarenka, and blasted nine aces against just two for the Belarusian, who won an impressive 10 of 15 points at the net when she stepped up the pressure on Williams. "Victoria, you played unbelievable," said Williams at the trophy ceremony.
"What a great match and what a great person. Vika is such a great opponent, she's such a great fighter. It was never over until match point," added Williams.
The top seed collected the $2.6 million top prize and pocketed an addition $1 million bonus for having won the U.S. Open run-up series of tournaments.
Azarenka said she had been beaten by the better player.
"It is a tough loss. But the best player deserves the win today. I gave it all again this year," said