Second seed Azarenka will face Italy's unseeded Flavia Pennetta, whose brilliant run at Flushing Meadows has carried her to her first Grand Slam semi-final.
Should form prevail, Williams and Azarenka would set up the first repeat final at the US Open since Serena Williams played her sister Venus Williams for the prize in 2002.
Azarenka, 24, joined the trio of 31 year olds with a last-eight victory over 30-year-old Daniela Hantuchova of Slovakia in a women's draw that has underlined the fitness and ferocity of the old guard.
Should they lift the trophy on Sunday, Williams, Li or Pennetta would become the oldest US Open women's winner since tennis turned professional in 1968, supplanting Australian Margaret Court, who was 31 years and 55 days old when she triumphed in 1973.
Williams has been on fire, losing only 13 games and spent the least time on court although she has done double duty playing with sister Venus in the women's doubles.
The American aced her quarter-finals test against Spain's Carla Suarez Navarro by delivering a 6-0 6-0 double bagel win.
Williams later offered a shuddering thought.
"I like to believe there is always room for improvement," she said.
Against former French Open winner Li, current French Open champion Williams holds an 8-1 advantage in head-to-head meetings, but has had some hard battles along the way. She won their last encounter in Cincinnati 7-5 7-5 in the semis.
Williams holds the Chinese player in high regard.
"She's a great challenge," Williams said. "She moves really well. She does everything well.
"We've always played really close. Maybe next time we play she might want to go from close to a win."
Li has been on a revenge tour, beating Briton Laura Robson to avenge a 2012 US Open loss, and Serb Jelena Jankovic, who had enjoyed a four-match winning streak against her.
"It's a good challenge to play against her, because I think we always have tough match, even in Cincinnati," said Li.
"There are only four players at end of the tournament, so nothing you can worry about. Just play the tennis. Just enjoy the time."
Azarenka and Pennetta have split their two previous matches, missing a crack at a rubber match when the Belarusian withdrew from their second-round Wimbledon contest this year because of a knee injury.
Pennetta, like Williams, has not dropped a set during her US Open run and is back to playing at her best level after having her 2012 season curtailed by a wrist injury that kept her out of the US Open and required surgery.
"I'm confident because I'm here," said Pennetta, who ousted fourth-seeded compatriot Sara Errani, former champion Svetlana Kunznetsova and red-hot Simona Halep along the way.
"I really don't feel this pressure. In Italy we already have one champion in Grand Slam," she said, referring to 2010 French Open winner Francesca Schiavone.
"I really don't feel this pressure right now."
Azarenka, who overcame former number one Ana Ivanovic 4-6 6-3 6-4 in the fourth round, said she would not underestimate the Italian despite her ranking of 83.
"Flavia is such an amazing person," said Azarenka. "To see her go through the injuries and coming up with the best results she has right now, it's really amazing.
"She's a very good player, an all-around player. She can do anything. She has great touch, great variety. She can create power, create spin. She's also very experienced and has been in the top 10.
"Her ranking doesn't matter. It's about the moment right now, and she's playing terrific tennis."
