STUTTGART, Germany - Anett Kontaveit rolled through the first eight games of her second round encounter with two-time Grand Slam champion Angelique Kerber before the former World No.1 was forced to retire with a right thigh injury, trailing, 6-0, 2-0.

"She wasn't 100%, but I'm really happy to be in the quarterfinals," Kontaveit said during her on-court interview. "It's not the way I wanted to win today, but I'm still happy to be through."

Kontaveit saved two match points to defeat 2017 runner-up Kristina Mladenovic in a third-set tie-break less than 24 hours ago, but instead it was Kerber who appeared to be hampered physically from the start, taking a medical time-out at 5-0 in the opening set.

"I was feeling it a lot in the warm-up," Kerber said after the match. "It started a few days ago, but this morning, I was feeling it more and more. I was trying everything to go out there. I'm here in Stuttgart, my home tournament, so I was trying everything, thinking I could play for the fans.

"At the end, it was too much and I started to feel the pain a lot in the first two games, where maybe I was sliding a little bit differently."

The German captured the Stuttgart crown in 2015 and 2016, and fought valiantly to continue with a heavy right thigh strapping before ultimately deciding not to play on as the second set got underway.

"It's more because of the pain. I played a good match on clay court yesterday, so I'll try to take that feeling back home with me, so I can learn from yesterday's match. I'll try to forget today as soon as possible, but I did my best to go out there in front of my home crowd.

"Now, we have to see what is going on: how bad it is and how long it will take to be 100% again."

Still, Kontaveit never took her foot off the proverbial gas pedal, striking 13 winners to five unforced errors and converted four of five break point opportunities throughout the match.

"I think the level is very even, and anyone can beat anyone."

Up next for the Estonian is Anastasia Pavlyuchenkova, who won the first set over No.2 seed Garbiñe Muguruza before the Spaniard was also forced to reitre.

"I'm just taking it match by match. I'm really happy with the way I've been playing, and I'm just looking forward to the next match."