Osaka, who announced a surprise split from her coach Sascha Bajin earlier this month, was beaten 6-3 6-3 in the second round, having received a first-round bye as the top seed.
The 21-year-old, who won the Australian Open last month and the US Open last year, gave no reason for ending her successful partnership with Bajin.
— WTA (@WTA) February 19, 2019
Osaka enjoyed a remarkable rise up the world rankings after first linking up
with Bajin in the 2017 off-season.
The Japanese player climbed from 72nd to reach the top five during the course of 2018, when she won her first grand slam title, and then hit top spot after her success in Melbourne.
Bajin, who has previously worked as hitting partner for Serena Williams,
Caroline Wozniacki, Sloane Stephens and Victoria Azarenka, was named WTA coach of the year in 2018.
He responded to Osaka's tweet announcing the split by replying: "Thank you
Naomi. I wish you nothing but the best as well. What a ride that was. Thank you
for letting me be part of this."
Second seed Petra Kvitova, who lost to Osaka in the Australian Open final, had to come from a set down to beat Russia's Katerina Siniakova 6-7 (3) 6-4 6-4, while third seed Simona Halep defeated Eugenie Bouchard 7-6 (4) 6-4.
There were also victories for Garbine Muguruza, Angelique Kerber and Elina Svitolina, but seventh seed Kiki Bertens lost in three sets to Viktoria Kuzmova.
In the Hungarian Ladies Open, Kateryna Kozlova thrashed seventh seed Johanna Larsson 6-1 6-1 and Iga Swiatek dropped just one more game in beating Olga Danilovic 6-3 6-0.