Garbine Muguruza confirmed that a recurring back injury was to blame for her withdrawal from the Stuttgart Open on Thursday.
The Spaniard did not appear hampered in her movement despite losing the first set of her second-round match against Anastasia Pavlyuchenkova after dropping her serve in game 12.
But before the players returned to the court for the second set, Muguruza rose from her chair and shook hands with her opponent before leaving the arena.
Muguruza's retirement did not go down well with the Stuttgart crowd, who shortly beforehand had seen home favourite Angelique Kerber bow out with a thigh injury at 6-0 2-0 down to Anett Kontaveit.
"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 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."
"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," added Pavlyuchenkova.
"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 quarter-finals."
Like Muguruza, Kerber revealed that she had also come into the tournament with a pre-existing condition.
"I was feeling it a lot in the warm-up," said the 2015 and 2016 champion. "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.
"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 per cent again."
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