World tennis No.1 Naomi Osaka has been beaten in her first match since winning last month's Australian Open then splitting with her coach last week.
The Japanese star went down 6-3-6-3 to France's Kristina Mladenovic at the Dubai Duty Free Tennis Championships
Osaka struggled with her service game, winning only 48 per cent of her first serve points, and made 25 unforced errors, folding in just over an hour.
Mladenovic hit three fewer winners than Osaka's 19, but made nine unforced errors and converted seven of the 10 break point opportunities she carved out.
"It means a lot, honestly," Mladenovic said.
"I know I'm capable of beating great players. I did that in the past exactly on this court."
The Frenchwoman, who is ranked 67th in the world, faces Spain's Carla Suarez Navarro in the round of 16.
Suarez Navarro beat China's Shuai Zhang earlier on Tuesday.
Osaka's shock exit was the most dramatic result on a day in which most of the other big names in action progressed safely to the third round.
Australian Open runner-up Petra Kvitova rallied from a set down to beat Czech compatriot Katerina Siniakova 6-7 (3-7) 6-4 6-4, while third seed Simona Halep advanced with a 7-6 (7-4) 6-4 victory over Canadian wildcard Eugenie Bouchard.
Kvitova, the second seed, was almost the architect of her own downfall with nine double faults in the match, but recovered to set up a clash with American qualifier Jennifer Brady.
Brady pulled off another of the day's upsets when she beat 14th seed Caroline Garcia of France 6-4 7-5, joining compatriot Sofia Kenin in the next round.
Kenin ousted last year's Dubai runner-up Daria Kasatkina of Russia 6-3 2-6 6-4 and Alison Riske beat France's Alize Cornet 6-2 6-3 to mark another good day for the Americans in Dubai.
Defending champion Elina Svitolina also advanced after her second-round opponent Ons Jabeur of Tunisia was forced to retire with an apparent shoulder injury while trailing 7-6 (7-4) 4-0.
