Andy Murray may have kept Serena Williams waiting in New York, but the 23-time grand slam champion said she was "rooting" for her former doubles partner in his epic five-set comeback at the US Open.

Prior to Williams beating Kristie Ahn in straight sets on Tuesday, Murray needed almost five hours to get past Yoshihito Nishioka 4-6 4-6 7-6 (7-5) 7-6 (7-4) 6-4 on Arthur Ashe Stadium at Flushing Meadows.

In his first singles grand slam match since the 2019 Australian Open due to hip surgery and the coronavirus pandemic, former world number one and three-time major champion Murray rallied to stun his Japanese opponent in the first round.

While Williams' scheduled match was pushed back, the American superstar hailed Murray after the pair teamed up in the mixed doubles at last year's Wimbledon.

"Usually when you're waiting for a match, someone is down two sets, you root for the person that's ahead so you can get on the court and get off," Williams told reporters.

"I was rooting for Andy the whole time. I really wanted him to win. Gosh, when he was down in the third set, I was like, All right. I was just rooting for him so hard.

"I saw him give the racquet to his trainer. There's Andy, he plans on playing five sets here. I was really happy for him.

"I love his grit. I've always loved that, way before we played doubles. I always said he reminds me a lot of myself. I'm just a big fan.

"It was really good because I know what it's like to be down, I know what it's like to be injured, I know what it's like to be counted out. I felt like it was a real gutsy win for him and I was really happy."

Williams – without a major title since winning the 2017 Australian Open – started her latest bid for a 24th grand slam crown with a 7-5 6-3 victory over fellow American Ahn.

The third seed and six-time US Open champion also broke the record for most singles wins (102) in the tournament's history, surpassing Chris Evert.

"In a weird way I feel like every time I come here I'm being told I broke another record," Williams said. "I felt like I had something last year. Maybe it was a tie for Chris Evert.

"But it's cool. I don't think I appreciate it enough, which is unfortunate. But I'm in the middle of a grand slam, so it's not the time to be focused for me on records when I'm thinking about winning a tournament."

This year's US Open is a slam like no other, played behind closed doors and with strict measures in place due to COVID-19.

"I think what's most important about this event taking place is just the spirit," the 38-year-old added. "Sport has been gone for so long, particularly tennis. We missed two grand slams. The US Open is the first major tennis event since Australian Open.

"The morale can be really low in the world with everything that's going on. Sometimes you just want to take your mind off. People have been doing that for generations through sport.

"That's one of the reasons I was so supportive of the US Open. I felt like it was such a good time to get back out there for athletes and for fans to kind of just disconnect and be a fan, and for athletes to do what they do best."