The Duchess of Sussex caused a fashion “racquet” at pal Serena Williams’ ill-fated quest for a record-tying 24th Grand Slam singles title at the U.S. Open on Saturday, rocking a denim J. Crew dress that has since sold out online.

The royal was a loyal cheerleader in Williams’ courtside box at Arthur Ashe Stadium in Queens, New York, sitting with the tennis star’s husband Alexis Ohanian, other family members and Vogue editor Anna Wintour.

Meghan, Duchess of Sussex, attends the Women's Singles final match between Serena Williams of the United States and Bianca Andreescu of Canada on day thirteen of the 2019 US Open at the USTA Billie Jean King National Tennis Center. (Photo by Clive Brunskill/Getty Images)

The new mom smiled, cheered and even appeared to pray as her friend battled 19-year-old Bianca Andreescu on the court, ultimately losing in a 6-3, 7-5 finish.

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Style blog Meghan’s Mirror quickly identified the duchess’ duds as a $118 belted shirtdress and $148 gray sweater blazer, both by J.Crew. According to the site, the dress has sold out, while the sweater blazer was recently restocked.

The former actress landed in the Big Apple on Friday for the “last-minute trip” on a commercial flight from London to cheer on Williams, a source said. (Photo by Clive Brunskill/Getty Images)

The preppy American brand has long been one of Duchess Meghan’s go-to choices for casual outings, Marie Claire reports, while the “Markle effect” has allegedly caused items to fly off the store's shelves before.

The new mom smiled, cheered and even appeared to pray as her friend battled 19-year-old Bianca Andreescu on the court, ultimately losing in a 6-3, 7-5 finish. (Photo by Mike Stobe/Getty Images)

Markle was also spotted fiddling with her delicate gold necklace during the match, a piece that Cosmopolitan reports paid tribute her growing family. According to the outlet, the necklace featured initial tags for “H” and “A” – honoring husband Prince Harry and baby son Archie.

Style blog Meghan’s Mirror quickly identified the duchess’ duds as a $118 belted shirtdress and $148 gray sweater blazer, both by J.Crew. (J.Crew)

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The former actress landed in the Big Apple on Friday for the “last minute trip” on a commercial flight from London to cheer on Williams, a source said.

Serena Williams congratulated Bianca Andreescu after losing to Andreescu in the women's singles final of the U.S. Open tennis championships. (AP Photo/Eduardo Munoz Alvarez)

It's also a quick solo getaway before the Sussex crew embarks on their first royal family trip to Africa between Sept. 23 and Oct. 2.

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Fox News’ Jessica Napoli contributed to this report.