Williams' close friend Meghan, the Duchess of Sussex, is reportedly on her way from London to New York to watch Williams play 19-year-old Canadian star Bianca Andreescu.
Meghan, 38, left Prince Harry and their 4-month-old son Archie at home in Windsor to catch a last-minute flight to New York, according to The Times of London.
Duchess Meghan, who faced scrutiny for traveling with Harry and Archie this summer on private jets, took a commercial flight, according to the Times.
Buckingham Palace declined to comment but also did not deny the Times' report.
Meghan and Williams have been friends since meeting at the 2010 Super Bowl in Miami.
The tennis star and her husband, Reddit co-founder Alexis Ohanian, attended Meghan and Harry's wedding last May.
Williams was also one of the organizers of Meghan's star-studded baby shower in New York City earlier this year.
Last year, after Meghan's pregnancy announced, Williams, mom of 2-year-old Olympia, said the two friends had been "relying on each other a lot recently."
Meghan watched Williams play at Wimbledon in July, including sitting court side with her sister-in-law Duchess Kate to watch Williams play in the women's final.
"She couldn’t be a better friend to me," Williams said of Meghan at Wimbledon. "Low moments, high moments, she’s always there and that’s all I want to be to her."
Williams lost in the Wimbledon final to Simona Halep. With the U.S. Open, she is continuing her bid to win her first Grand Slam since becoming a mom.
A win on Saturday for Williams against Andreescu would also be the 24th Grand Slam singles title for William, which would tie her with Margaret Court, who has held the women's record for decades.
Meghan's last-minute trip to New York comes as she is preparing for her family's first official tour. She, Harry and Archie will embark on a 10-day tour of South Africa on Sept. 23.
The duchess will also mark her return from maternity leave next week with the launch of the capsule fashion collection she created to benefit the Smart Works charity.
That launch will take place in London next Thursday, Sept. 12, according to Buckingham Palace.