STUTTGART, Germany -- Anastasia Pavlyuchenkova of Russia reached the quarterfinals of the Porsche Tennis Grand Prix when No.2 seed Garbiñe Muguruza of Spain retired from the match due to a lower back injury, after the Russian claimed the first set, 7-5.

It was a stunning turn of events after a closely fought opening frame which lasted 43 minutes. It was the third consecutive match on the Center Court in Stuttgart which ended in retirement following the injuries to teenager Marketa Vondrousova and two-time champion Angelique Kerber.

"It was a surprise for me as well, because I think we both started the match quite well, and I didn’t know she was hurt," Pavlyuchenkova told the press, after the match. "I felt like I was playing good, and I was in the zone. That’s not the best way to win the match, but it is how it is, and I’m so very happy to be in the quarterfinals."

Muguruza also spoke with the press after her retirement. "I’m disappointed, because this tournament, I never do really great results, and to retire, it’s always bad for my Muguruza team," said the reigning Wimbledon champion. "I’ve been feeling my lower back since the Fed Cup, more or less since I started playing on clay, and I rushed a little bit here." 

Despite the abrupt ending, it was an excellent performance by the World No.27. The big-hitting Russian used her powerful groundstrokes to great effect in the set, hitting 15 winners to only 12 unforced errors, and breaking the Muguruza serve twice while only ceding her own serve once.

Pavlyuchenkova was bolstered by her hard-fought three-set win over Madison Keys in the opening round. "I felt very confident and good out there tonight, especially after yesterday’s match, because it was super close and a tough match," she said. "I guess that gave me extra motivation and confidence as well, that I’m able to produce this tennis for almost three hours."

The match started with both hard-hitting players in great form. Pavlyuchenkova hit an ace to reach game point en route to a hold for 1-0, while Muguruza slammed a backhand winner past the Russian to level at 1-1.

But, after an overpowering hold at love to lead 4-3, Pavlyuchenkova seemed to grab the critical game of the set when she broke the Spaniard with a searing forehand winner down the line, to lead 5-3.

However, when serving for the set in the next game, the Russian hit two consecutive double faults to hand Muguruza triple break point. Pavlyuchenkova fended off the first two chances, but pushed a backhand long on the third to let Muguruza back on serve at 5-4.

"After 5-3, I started to rush a bit, maybe because I wanted to get things done quickly!" Pavlyuchenkova declaimed with a laugh. "That’s why I missed some extra, unnecessary shots. It was not easy, also, to finish the set."

It was at this juncture, though, that Muguruza received a medical timeout for her lower back, and although the World No.3 held for 5-5, she would shake her head after points, even victorious ones, giving the impression that she was struggling severely.

Pavlyuchenkova continued to press aggressively, holding onto a tough service game to lead 6-5. The Russian seized the moment in the following game, hitting a backhand winner into the corner to reach 15-30, and followed with a forehand winner down the line to bring up double set point.

A forehand error by Muguruza on the first set point gave Pavlyuchenkova the opener, and after that, Muguruza determined she was unable to continue any further. The Spaniard walked over to Pavlyuchenkova to shake hands, placing the Russian into her first quarterfinal of the 2018 season.

Pavlyuchenkova will now take on Estonia's Anett Kontaveit in the quarterfinals, after Kontaveit moved through to the final eight when Kerber retired against her while trailing 6-0, 2-0.

"Of course, [Kontaveit's] a tough opponent, and she’s been playing really good tennis out here," said Pavlyuchenkova. "I’ve been watching some of her matches, especially this crazy match yesterday she had against Kiki [Mladenovic]. She’s a good hitter, she will hit hard every ball, and I’ll be ready."