World No 3 Coco Gauff crashed out in the second round of the WTA 1000 tournament in Dubai on Tuesday when she was beaten 6-4, 7-5 by fellow American McCartney Kessler.
The world No 53, who beat Doha champion Amanda Anisimova in the first round, broke Gauff's serve three times and saved five of the six break points she faced.
The only time she slipped up came when she tried to serve out the match for the first time at 5-4 in the second set.
It is the first time that Kessler, 25, has beaten a top 10 player although she has won the first two titles of her career in the last six months - in Cleveland in August 2024 and then in Hobart at the beginning of January.
She is through to the third round of a WTA 1000 for the first time in her career, and will face No 14 seed Karolina Muchova who beat 2021 US Open winner Emma Raducanu 7-6 (8/6), 6-4.
Gauff made a good start to the year winning all her matches for the United States as they won the United Cup and reaching the quarterfinals at the Australian Open.
However, she lost in the second round of the Doha WTA 1000 last Tuesday, also in two sets, to Ukrainian Marta Kostyuk and has now fallen to world No 53 Kessler.
There were no such problems for the top two seeds.
World No 1 Aryna Sabalenka picked up her first win since the Australian Open semifinals with a 6-3, 6-4 victory over qualifier Veronika Kudermetova.
"It's always like a roller coaster of a game with (Kudermetova)," said Sabalenka.
"That's why I'm happy with the way I stayed focused today. No matter how she played, I was there fighting, trying to stay focused on myself and on my game."
Iga Swiatek crushed Victoria Azarenka 6-0, 6-2.
Jasmine Paolini kicked off her title defence with a win although the rain made her wait.
The Italian reached match point before rain forced the players off the court. Over four hours later, the world No 4 returned to win the final point, beating German qualifier Eva Lys 6-2, 7-5.