On Sunday, the upset bug at Arthur Ashe Stadium found its latest victim: defending U.S. Open champion, Coco Gauff.

Emma Navarro defeated the top-seeded American in three sets, sending her home early for the second consecutive Grand Slam.

Gauff's loss marks the end of a challenging summer hard-court swing for the No. 3 seed, who failed to advance beyond the second round in the two leadup events to the U.S. Open.

Here are the top reactions from Sunday's action.

Sunday U.S. Open Results

Men's Singles

No. 4 Alexander Zverev def. Brandon Nakashima, 3-6, 6-1, 6-2, 6-2

No. 9 Grigor Dimitrov def. No. 6 Andrey Rublev, 6-3, 7-6(3), 1-6, 3-6, 6-3

No. 12 Taylor Fritz def. No. 8 Casper Ruud, 3-6, 6-4, 6-3, 6-2

No. 20 Frances Tiafoe def. Alexei Popyrin, 6-4, 7-6(3), 2-6, 6-3

Women's Singles

No. 2 Aryna Sabalenka def. No. 33 Elise Mertens, 6-2, 6-4

No. 13 Emma Navarro def. No. 3 Coco Gauff, 6-3, 4-6, 6-3

No. 7 Qinwen Zheng def. Donna Vekic, 7-6(2), 4-6, 6-2

No. 26 Paula Badosa def. Yafan Wang, 6-1, 6-2

Taylor Fritz First American Since 2007 to Reach Three Grand Slam QF in Calendar Year

Taylor Fritz defeated Casper Ruud in four sets, becoming the first American since Andy Roddick in 2007 to reach three major quarterfinals in a calendar year, according to ESPN.

The American was nearly unstoppable on serve, hitting 24 aces and winning 84% of first serves while losing just one of his 18 service games.

While Ruud typically thrives in baseline rallies, as the match progressed, Fritzz began taking more risks, shortening points with aggressive yet consistent forehands that were simply too much for the Norwegian.

The No. 12 seed hit a total of 56 winners, including a near flawless fourth set, where he hit 18 winners compared to just one unforced error.

Fritz has now played three straight consistent matches, dropping just one total set.

On Tuesday, he'll face either No. 4 seed Alexander Zverev or Brandon Nakashima, looking to reach his first career Grand Slam semifinal.

Grigor Dimitrov Reaches First U.S. Open Quarterfinal Since 2019

For the first time since 2019, Grigor Dimitrov has reached the quarterfinal round of the U.S. Open.

The top-seeded Bulgarian defeated Andrey Rublev in a five-set thriller, becoming the first player born in the 1990s to reach 300 career hard-court wins.

The former U.S. Open quarterfinalist quickly established a two-set-to-love lead before Rublev's serve began to cause him all sorts of problems.

In the third and fourth sets combined, Rublev won 24 of 28 points on his first serve.

But the final set was all Dimitrov's.

The No. 9 seed grabbed a pivotal service break against his opponent early in the fifth set and then saved two break opportunities late, which likely would have opened the door for Rublev to get back in the match.

Coco Gauff Suffers Early Exit At U.S. Open

In her fourth round loss, Coco Gauff's serving issues and inconsistency proved to be her Achilles heel.

The top-seeded American hit just 46% of her first serves, committing 60 unforced errors to just 14 winners.

In fact, her 19 double faults outnumbered her 14 winners.

Despite securing the second set after breaking Navarro's serve twice, Gauff's struggles on the first serve opened the door for her American compatriot in the third set.

At 1-1 in the third set, Gauff hit consecutive double faults, gifting a service break to her American compatriot.

In defeat, Gauff's hopes of defending her title have come to an end.

Meanwhile, Navarro has now reached quarterfinal appearances in back-to-back Grand Slams with a victory and will face Paula Badosa on Tuesday.