Four-time Grand Slam champion Naomi Osaka's poor form continued on Tuesday at the Bad Homburg Open as she fell 6-4, 6-4 against American No. 5 seed Emma Navarro.

Navarro will face compatriot Jessica Pegula on Thursday, after the top seed powered past Czech Katerina Siniakova 6-2, 6-3 earlier.

Osaka had won her first match of the grass season in the first round with a victory in straight sets over Olga Danilovic.