Naomi Osaka endured a tough start to life as world number one as she was dumped out of the Dubai Tennis Championships by Kristina Mladenovic.

The 21-year-old climbed to the top of the rankings for the first time after winning a second consecutive grand slam title at the Australian Open at the start of the year, yet her form deserted her as she returned to action on Tuesday.

Playing without a full-time coach following Sascha Bajin's departure - instead aided by Japan Tennis Association trainer Masashi Yoshikawa - Osaka floundered early on and could not recover.

Mladenovic ran out a 6-3 6-3 winner and upset the assembled Japanese fans who witnessed a surprisingly flat performance from the evening's star attraction.

Osaka was slow to get going in front of an expectant crowd and was quickly broken twice, first finding the net after a series of sloppy serves and then bowing again under pressure from Mladenovic.

The Japanese responded to the second setback in style, breaking back after a pair of stunning returns brought her fans to their feet, yet a double fault at the end of another disappointing service game handed Mladenovic the opening set.

Osaka had her head in her hands as she dropped three consecutive points and received a time violation at the start of the second, swiftly suffering another break at the hands of a now rampant opponent.

That was the first of seven breaks in succession, with Osaka returning well but repeatedly giving up her foothold in the match, shrugging bemusedly as she failed to hold serve for the seventh time in the set.

Mladenovic then had the opportunity to serve out the match and she assuredly delivered to clinch victory inside 65 minutes.