Dubai-based Frenchwoman Kristina Mladenovic claimed a 6-3, 6-3 victory to reach the third round as Osaka made 25 unforced errors.
The rout, which featured seven breaks of the Osaka serve, took barely an hour, sending Mladenovic into a Wednesday contest with Carla Suarez Navarro.
"I played pretty bad, you have days like that. You just have to find a way to, like, win no matter what the situation is," Osaka said.
"(My serve) was a disaster. I haven't been practising well recently, it would go away during the matches. I was kind of counting on that -- it didn't happen.
"But even if I don't win any matches for the rest of the year, I wouldn't say I'm concerned. I think I'm pretty young. I still feel like I have a lot to learn.
"After I lose a sort of bad match, I'm just looking forward to the next tournament."
The second set was riddled with seven consecutive breaks of serve before Mladenovic finished off the job with a winner to the empty court on match point.
Osaka stunned the tennis world last week by announcing her split from coach Sascha Bajin despite winning back-to-back Grand Slam titles at the US and Australian Opens.
She said earlier this week that she would not prioritise "success over happiness" when asked about the sacking, and she never looked like winning in Dubai as she also served five double faults.
Earlier on Tuesday, Simona Halep and Petra Kvitova both won their opening matches, while Elina Svitolina made a strong start to her bid for a third straight title.
Kvitova, the 2013 winner, recovered from losing a first-set tiebreak to defeat Katerina Siniakova 6-7 (3/7), 6-4, 6-4 and squeeze into the third round.
Halep, Dubai champion in 2015, saw off 2014 Wimbledon finalist Eugenie Bouchard 7-6 (7/4), 6-4.
The Romanian said she is still feeling the effects of her weekend final defeat in Doha by Elise Mertens.
"It was a good match because I won it. I tried to finish the points faster because I'm a little bit tired," Halep said.
"She played well, the level was pretty good. At some points I really ran very well. I'm confident. I feel good on court even if I'm tired."
A third former world number one also advanced as Angelique Kerber beat Dalila Jakupovic 7-6 (7/4), 6-3.
Kvitova, Wimbledon champion in 2011 and 2014, said she was unfazed about the early midday start to her match.
"I don't really care if I play the first match," the Australian Open runner-up said. "Now I'm done, so I have a free afternoon.
"I'm not sure what I'm pleased with... maybe with my comeback at the end."
- Svitolina benefits from Jabeur injury -
Local favourite Ons Jabeur was forced to retire with the Tunisian trailing Svitolina 7-6 (7/4), 4-0.
Jabeur took a fall late in the opening set, rolling her ankle while also hitting her shoulder.
She took a medical timeout out but was only able to carry on in the second set for four games.
Third seed Svitolina is hoping to become the first women to win three Dubai titles.
"That is a big motivation for me here," the Ukrainian said. "Today I didn't really feel in a match mood, I don't know why.
"I was struggling a bit, I had a very slow start. Hopefully tomorrow will be better.
"I told myself that I'm here to make history of the tournament."
Elsewhere, fourth seed Karolina Pliskova continued her good run of form after reaching the semi-finals at Melbourne Park by edging out former Australian Open finalist Dominika Cibulkova 6-2, 3-6, 6-3.