Serena Williams says she would not have believed after her first US Open title in 1999 that she would be playing in a record 10th final 20 years later.

Williams, 37, stormed into the Flushing Meadows final with a 6-3 6-1 win over Ukrainian fifth seed Elina Svitolina.

The American will bid for a record-equalling 24th major singles title on Saturday against Bianca Andreescu.

"I would have thought it was a sick joke," Williams said of being in a final 20 years after her first.

"At 17, I thought for sure I'd be retired at 28, 29, living my life. I would definitely not have believed somebody saying that."

Williams is aiming to equal Australian Margaret Court's all-time record of Grand Slam singles titles.

The eighth seed has competed in three major finals since returning from giving birth to daughter Olympia in September 2017. She lost in the past two Wimbledon finals as well as a controversial US Open showpiece against Naomi Osaka in New York last year.

"I think it's cool that I've been in more finals than I think anyone on tour after being pregnant. That's kind of awesome," she said.

"I look at it that way because it's not easy to go through what I did and come back, and so fast.

"To keep playing, to also not be 20 years old, I'm pretty proud of myself."

Williams is the favourite to beat 19-year-old Canadian Andreescu, who will be competing in her first Grand Slam final after beating Switzerland's Belinda Bencic in the last four, particularly after the way in which she destroyed semi-final opponent Svitolina.

Williams hit 33 winners in a powerful display that Svitolina could not cope with, although Williams agreed with the Ukrainian's assertion she could have played even better.

"I think it was solid. It definitely wasn't my best tennis," Williams said.

"It's interesting that she knows that. She's a super professional to know that.

"She probably could have played better as well. I definitely know I could have played better."