World number one Williams will be shooting for her 17th grand slam singles title and fifth U.S. crown when she defends against two-time reigning Australian Open champion Azarenka, who fell 7-5 in the third set to Serena in last year's Flushing Meadows final.
Williams said she and Azarenka have no trouble getting their game faces on for a showdown.
"We completely get along, and once the match is on we are completely opponents," said Williams. "That's what it's about.
"We leave everything on the court, play as hard as we can, almost as if we've never met each other in our lives."
Azarenka said Williams brings out the best in her.
"She's obviously an amazing player. She's the greatest of all time," said the 24-year-old Belarussian.
"When you play against Serena, you have to play your best because she makes me play my best.
"I think I kind of do the same way to her. I think we kind of raise each other's level all the time and take each other to the limit - to go out of that place where you sometimes don't know how far you can go, but you just still go for it."
Williams leads their career head-to-head 12-3, but Azarenka is gaining ground with wins in two of their last three meetings.
The world number two beat the American in the final in Cincinnati, taking the U.S. Open run-up event by 8-6 in a third-set tiebreaker, and in the final this year at Doha.
"You've got to fight," Azarenka said about taking on Williams. "You've got to run, you've got to grind, and you've got to bite with your teeth for whatever opportunity you have."
Williams, a 6-0 6-3 winner against China's Li Na in the semi-finals, has been overpowering in the tournament, losing a mere 16 games from six straight wins.
Azarenka has been more uneven, dropping two sets along the way and struggling at times to hold serve, but she looked to be approaching top form with her 6-4 6-2 demolition of Italian Flavia Pennetta in the semi-finals.
Williams said she expected Azarenka to raise her level when they face off in Arthur Ashe Stadium.
"I definitely feel like when she plays me she plays her best, by far. I have seen her play other players, and when I play her I'm playing a totally different player," Williams said. "Obviously, she brings her best game."
Williams shrugged off her recent results against Azarenka, saying the grand slam stage was an entirely different matter.
"Different energy, different opportunities. This is for a grand slam," underlined Williams. "I mean, she's trying to win yet another one, I'm trying to win one myself."
Azarenka has yet to beat Williams in a grand slam event, losing all seven of their previous meetings.
Williams, who won her first slam singles 14 years ago by defeating Martina Hingis at the U.S. Open, has become increasingly aware of her status on the all-time lists, saying that she measured greatness by the numbers.
A 17th grand slam title would put her just one shy of two of her tennis heroes - Chris Evert and Martina Navratilova, who share fourth place on the all-time list of most grand slam singles trophies.
"I thought about that, but I can't think about that," said Williams about her quest to reach 18. "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."
A win for Williams, who turns 32 this month, would put her in the record books as the oldest U.S. Open women's champion since tennis turned professional in 1968, eclipsing Australian Margaret Court, who was 31 years and 55 days old when she triumphed in 1973.
Victory will be extra lucrative for Sunday's winner.
Besides the $2.6 million prize for claiming the title, Williams would receive a $1 million bonus for having won the U.S. Open series of run-up events, while Azarenka would pocket an additional $500,000 with the win for being the Open series runner-up.
(Editing by Gene Cherry)