As Serena Williams took to the court Saturday in the U.S. Open women's final in New York, she had a royal fan cheering her on from the stands.

Williams' close friend Meghan, the Duchess of Sussex, took a last-minute flight from London to New York to watch Williams play against 19-year-old Canadian Bianca Andreescu.

Meghan left Prince Harry and their 4-month-old son Archie at home in Windsor to see Williams. The tennis star is vying for her record-tying 24th Grand Slam title and her first since giving birth to daughter Olympia two years ago.

Meghan was seated between Patrick Mouratoglou, Serena’s coach, and Serena’s mother Oracene Price, at Arthur Ashe Stadium in Flushing Meadows, New York.

Serena's sister Venus Williams and Anna Wintour, the editor of Vogue, were seated behind them.

Meghan and Williams have been friends since meeting at the 2010 Super Bowl in Miami.

Williams 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 said the two friends had been "relying on each other a lot recently."

Meghan watched Williams play at Wimbledon in July, including sitting courtside with her sister-in-law Duchess Kate to watch Williams play in the women's final. Williams lost that match to Simona Halep.

"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."

Meghan's last-minute trip to New York comes as she is preparing for her family's first official tour. Meghan, Harry and Archie will embark on a 10-day trip to 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.

ABC News' Joshua Hoyos contributed reporting.