World number one Naomi Osaka began her clay-court season with a straight-set victory over Hsieh Su-wei at the Porsche Tennis Grand Prix in Stuttgart.

The Japanese, who lost to Taiwan’s Hsieh at the Miami Open in March, won 6-4 6-3 to reach the quarter-finals.

The victory ensures Osaka will retain the world number one ranking for at least another week.

Osaka will face Donna Vekic next after the Croat overcame Russia’s Daria Kasatkina 6-1 7-5.