REUTERS: World number one Naomi Osaka was beaten in her opening match at the Dubai Duty Free Tennis Championships on Tuesday, losing to France's Kristina Mladenovic 6-3 6-3 on her return to competitive action after winning last month's Australian Open.
Osaka, who parted ways with coach Sascha Bajin last week following her victory in Melbourne, struggled with her service game, winning 48 percent of her first serve points and making 25 unforced errors as she folded 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 en route to registering her first career victory over a reigning world number one.
"It means a lot, honestly," Mladenovic said after the match. "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) 6-4 6-4, while third seed Simona Halep advanced with a 7-6(4) 6-4 victory over Canadian wild card 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.
"Even (though) I lost the tiebreak, I was really fighting," Kvitova said. "I won my first two games in the second set. It was really helpful for me for the mentally. Then I'm glad that I was able to hold my serve until the chances in the third."
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(4) 4-0.
(Reporting by Simon Jennings in Bengaluru; Editing by Hugh Lawson and Christian Radnedge)