US Open 2021: Sports world reacts to Daniil Medvedev's stunning upset over Novak Djokovic

Medvedev wins US Open, stopping Djokovic short of Grand Slam (3:03)

Daniil Medvedev plays one of the best matches of his career to win the US Open in straight sets, denying Novak Djokovic the calendar-year Grand Slam. (3:03)

Well, that's one way to win your first major title -- by coolly thwarting one of the greatest to ever play the game. Daniil Medvedev became a champion on Sunday when he defeated Novak Djokovic 6-4, 6-4, 6-4 with history on the line in the 2021 US Open final.

Medvedev's upset win ended Djokovic's bid to break a three-way tie with Roger Federer and Rafael Nadal (20 apiece) for most major titles in men's tennis history.

But that's not all a Djokovic win would have signified. It also would have marked the completion of the calendar-year Grand Slam, making Djokovic the first man to win all four majors -- the Australian Open, French Open, Wimbledon and US Open -- in the same year since 1969. No big deal.

We were being facetious, of course. The possibility was an entirely big deal. Which is why it was no surprise that the stars were out in full force to watch Sunday's match in New York.

From Brad Pitt and Bradley Cooper to Spike Lee and Maria Sharapova, here's who was in attendance as Medvedev became the only player to defeat Djokovic in a Grand Slam in 2021:

As Medvedev put the finishing touches on his stunning upset, stars also took to social media to react to the remarkable moment: