Aryna Sabalenka was playing for the first time since winning the Australian Open, but lost to Croatia's Donna Vekic at the Dubai Tennis Championships.
Sabalenka has lost 12 of her last 13 matches as Croatia's Vekic won 6-7 (5-7), 6-3, 6-0 in the second round.
“From my point of view, the level was very poor,” said the Belarusian second seed.
World No. 1 Iga Swiatek, who won the title in Doha last week, defeated Sloane Stephens 6-4, 6-4, with third seed Coco Gauff also advancing.
Gauff, who turns 20 next month, defeated Italy's Elisabetta Cocciaretto 6-1, 7-5, but shutting out the US Open champion was no easy feat.
Service games were difficult for both players, and Gauff was unable to win at 5-4, but she quickly broke back at 6-5, ending six consecutive breaks and earning her second chance.
Gauff will face in-form Czech Republic's Karolina Pliskova in the last 16, while Swiatek will face Ukraine's Elina Svitolina for a chance to avenge her loss in the Wimbledon quarter-finals last year.
2022 Wimbledon champion Elena Rybakina will also be among the WTA stars competing in Tuesday's opening match, as the top eight seeds received byes in the first round in Dubai.
Rybakina won the title in Abu Dhabi and then finished runner-up to Swiatek in Doha before dropping the first set against Victoria Azarenka but winning the second before the Belarusian withdrew due to injury.
The fourth seed from Kazakhstan will next face Poland's Magdalena Flech, while Vekic's reward will be a match against Romania's Sorana Cirstea.
