At 31, the American is more dominant than she has ever been, and her 7-5 6-7 (6/8) 6-1 victory over Azarenka in New York gave her a ninth title of the year.
She has lost only four matches all season, but two of those have come in grand slams, where traditionally she has been at her strongest.
When Azarenka recovered from 4-1 down to take the second set with Williams having twice served for the match, it seemed the world number one may have to contemplate more slam disappointment.
But Williams responded superbly and won the third set easily to add the US Open to the French Open title she clinched in June.
She said: "I felt almost disappointed with my year, to be honest. I felt like, yeah, I won the French Open, but I wasn't happy with my performances in the other two slams, and not even making it to the quarter-finals of one.
"So I definitely feel a lot better with at least a second grand slam under my belt this year."
Williams' victory gave her a 17th grand slam singles title, moving her level with Roger Federer and only one behind both Chris Evert and Martina Navratilova.
"It's an honour to be even with Roger," said Williams. "He's been such a great champion throughout the years, and he's just an unbelievable competitor and he's still playing, and he can probably still win more.
"He's just been so incredibly consistent, so we have had really different careers.
"Then to be compared with Chrissy and Martina - I can't necessarily compare myself to them, because numbers wise they're still greater."
There were high hopes the match would be as dramatic as last year's final between the pair, which Williams won after Azarenka had served for the title, and it did not disappoint.
Williams struggled with the wind in the first set especially, casting panicked glances at her box and appearing distracted by her billowing dress and hair.
But Azarenka could not capitalise, and it looked like Williams would cruise to victory when she moved 4-1 ahead with two breaks of serve in the second set.
Azarenka is as big a fighter as Williams, though, and she hung on grimly and then seized her chance when the American got tight.
Twice Williams served for the match and twice Azarenka broke back, and the Belarusian went on to take the tie-break on her third opportunity.
It seemed like it would be a tough set for Williams to bounce back from but the world number one refocused and pulled away to clinch victory after two hours and 45 minutes.
Williams said: "I obviously would have preferred to close it out in straight sets. But going against a great opponent like Victoria, you have to be able to realise that that can happen and you have to keep fighting for everything.
"I thought, 'I have to stay positive'. It was very easy to get down. I didn't want to get down on myself.
"I wanted to be a little strict with myself so I could play better. I wanted to do different things and try to have a better performance."
Williams admitted she did not deal with the wind as well as she might have, saying: "The wind was unbelievable. It just got worse and worse. It never let up.
"But at this point you have to be able to play under any circumstances. It wasn't very easy. I think I made a lot of errors because of that. I definitely could have moved my feet better."
Azarenka was in tears at the end but felt there was nothing more she could have done.
The world number two described the determination of both players as rising like boiling water, and she said: "It was a great match.
"There was no letdown. I think in the third set the momentum changed a little bit, and I kind of felt like I lost that momentum.
"But she really made it happen. In that particular moment she was tougher. She was more consistent and she deserved to win.
"I wish I could have done something better. I felt like I had opportunities in the first set as well. But it's okay. I did everything I could.
"The game overall, there are things that could have been better. But I gave my heart. I fought as hard as I could.
"So that's what is important for me, that I lost to a great champion. But I'm still going to have my head up."