Caroline Wozniacki, of Denmark, left, and Serena Williams, of the United States, pose for photos after Williams defeated Wozniacki in the championship match of the 2014 U.S. Open tennis tournament, Sunday, Sept. 7, 2014, in New York. (AP Photo/Darron Cummings)

NEW YORK – Moments after Serena WIlliams secured her 18th Grand Slam title on Sunday, her opponent, Caroline Wozniacki, was asked to say a few words. The ever-gracious Wozniacki, 24, took the microphone and congratulated her friend.

“You’re an unbelievable friend,” Wozniacki added, “and you definitely owe drinks later.” Everyone in the stadium laughed, and Williams, 32, later confirmed that they'd be celebrating together that night. Just after midnight, she posted photo proof of her and Caro out in Manhattan.

No word on who bought the drinks, but Page Six reports that the group of more than 30, which included Joe Jonas, was at VIP Room drinking Champagne and vodka and dancing until about 3a.m.

The next morning, Williams was back on Instagram to let fans know that she'd be skipping a different celebration: the annual visit to CBS's "Late Show with David Letterman." "Passed on this...lol," she wrote.

She also passed on NBC's "Today Show," and ABC's "Good Morning America" and "Live with Kelly and Michael." The shows teased her appearances, but then had to inform viewers that she had cancelled. Appearing on the morning show segments would've left almost no time to sleep. But she still didn't get much – by 10:30a.m., she was touring the Empire State Building and attending a J.P. Morgan event, commitments required of all U.S. Open champions.

Serena Williams holds the U.S. Open tennis women's singles championship trophy during a visit to the 103rd floor of the Empire State Building, Monday, Sept. 8, 2014, in New York. Williams defeated Caroline Wozniacki, 6-3, 6-3 on Sunday. (AP Photo/Charles Krupa) More

As for the TV appearances, she's been through the morning show circuit at least five times, including the last two years in a row. She was on Letterman's show just before the Open, smashing windows with a tennis racket and playing tennis against the late-night host. She visits his show every year around the eve of the Open. With the championship secured, she seems to have chosen to thank her fans through social media instead of mainstream media.

She and Wozniacki attended a Chris Bosh event that night, and Williams was right back to work on Tuesday: She traded the tennis court for the runway, hosting her first fashion show as part of New York Fashion Week.

After the show, Caro was once again by her side.

Their friendship had been a hot topic throughout the two-week U.S. Open. One reporter couldn't get the word "gal-pal" out of his head. Others called them "besties." But repeatedly, Williams reminded them that playing a friendly opponent is nothing new to her.

"If I can play Venus, I can play anyone," Williams said. Once the final was set, Williams posted two throwback photos on Instagram, of when they attended the NBA Finals together earlier this year. "The hat says it all," she wrote.

And as they promised, meeting in the finals had no impact on their friendship. The post-match celebrations may have hurt Williams's relationships with the TV networks and with fans hoping to see her on the morning shows, but obviously not with her bestie.

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