World number one Naomi Osaka has withdrawn from her Porsche Tennis Grand Prix semi-final against Anett Kontaveit with an abdominal injury.

The Japanese said she felt pain in her left abdomen during her victory over Donna Vekic in Stuttgart on Friday.

Osaka, 21, said she had to "roll out of bed" on Saturday morning and was unable to lift her upper body.

Estonian eighth seed Kontaveit will play either Petra Kvitova or Kiki Bertens in Sunday's final.

"It's an ab strain, something that I've had before," Osaka said.

"Thankfully, because I've had it so many times, I am able to tell what it is and I know what to do."

Florida-based Osaka will remain in Europe to have the injury assessed.