There's a reason why no tennis player, male or female, has been able to win a Grand Slam since Steffi Graf did it back in 1988. It's incredibly difficult to do, as the G.O.A.T. Serena Williams—who completed the "Serena Slam" for the second time earlier this year—just learned.

Despite being a huge underdog heading into her match against Serena, Italian tennis player Roberta Vinci defeated Serena 2-6, 6-4, 6-4 in the semifinals of the U.S. Open at Arthur Ashe Stadium this afternoon.

Vinci will now advance to take on Flavia Pennetta in the finals tomorrow.

This marks just the second time in 67 matches that Serena has lost to a player ranked outside of the top 10 at the U.S. Open. The only other time it happened was back in 2009 when she lost to Kim Clijsters in the semifinals. The loss to Vinci also snaps an incredible 26-match win streak at the U.S. Open that Serena started back in 2011.

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