Stuttgart - World No 1 Naomi Osaka battled back from 1-5 down in the final set to beat Donna Vekic 6-3, 4-6, 7-6 (7/4) on Friday to reach her first career clay court semi-final in Stuttgart.

"I just told myself I didn't want to come away from here with any regrets," said 21-year-old Osaka.

She said she was "stressed out" and "making excuses" before managing to turn the tables on Croatia's Vekic.

The dramatic comeback saw Osaka reach her first ever clay court semi-final, where she will face either Estonia's Anett Kontaveit or former world No 1 and two-time major winner Victoria Azarenka on Saturday.

Saturday's match will also be Osaka's first semi-final since winning the Australian Open in January, having suffered early exits in Dubai, Indian Wells and Miami.

Osaka cannot lose her number one spot in Stuttgart, but she will be looking to keep a healthy points lead over world No 3 Petra Kvitova, who also won her quarter-final on Friday.

Kvitova overcame a rocky start to beat Latvia's Anastasija Sevastova 2-6, 6-2, 6-3.

"It was really difficult today, but I am very happy to be in the semi-final. I feel good and healthy," said Kvitova.

She will face either the Netherlands' Kiki Bertens or Germany's Angelique Kerber in the semi-finals.

"If I play against Angie in the semi-final, it will be like a Fed Cup tie because the crowd will be behind her," said Kvitova.

Results from the WTA tournament in Stuttgart on Friday (x denotes seeding):

Quarter-finals

Naomi Osaka (JPN x1) bt Donna Vekic (CRO) 6-3, 4-6, 7-6 (7/4)

Petra Kvitova (CZE x3) bt Anastasija Sevastova (LAT x7) 2-6, 6-2, 6-3