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.
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.