Mary-Kate enjoyed the action seated next to her fiancé Oliiver Sarkozy. The pair have yet to confirm their engagement but in recent weeks have taken to entering the public eye.
Sat just behind German model Heidi, Mary-Kate and Heidi managed to find the time to catch up with each other while snacking on popcorn.
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Heidi had spent Sunday in Los Angeles with her four children and mother Erna before jetting to New York on Monday to catch up with the 28-year-old. Vito is an art dealer and the son of celebrated filmmaker Julian Schnabel, and Heidi kissed and cuddled him throughout their day at the tennis.
Similar to when the pair were spotted in August at the Paddle and Party for Pink fundraiser, Mary-Kate made sure to keep her engagement ring tucked away between herself and her older beau.
The ring, which was first spotted during Paris Fashion Week, is an intricate Cartier piece dating from 1953 — a fitting choice considering the blonde beauty's penchant for vintage clothing. It features a four-carat central diamond flanked by 16 sapphires, and the "petals" are set with single-cut diamonds.
Mary-Kate admitted she had considered making her own wedding dress.
"I'm, like, not even there yet," she said. "I think it may be a matter of getting five or six, or making two or three and being stressed out. That seems kind of far away. We'll take one step at a time."
Younger sister Elizabeth, who is engaged to Boyd Holbrook, also recently spoke about her own wedding plans, and when asked about whether she had picked a wedding date and whether it would coincide with her big sister Mary-Kate, said, "No, I just told her she gets to pick a date first!"