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."
|Giants WR Beckham Jr. explains why he might cramp up: ´I really don´t like water´|
|Man City pay training-pitch tribute to Joe Hart|
|Neymar to be rested as Tuchel rubbishes Ballon d´Or concerns|
|Sarri ´not interested´ in Christensen father´s quit threat|
|Cardinals fire offensive coordinator McCoy after 1-6 start|
|Neymar return unlikely, but anything can happen – Valverde|
|Bairstow a doubt for fourth ODI|
|Manchester United fined for late arrival and delayed kick-off against Valencia|
|Australia-Pakistan chat not ´especially polite´ - Sarfraz|
|Pogba relishing ´pressure game´ against in-form Hazard|
|Don´t bet on Cardiff survival, warns Warnock|
|Buffon enjoying Tuchel´s methods at PSG|
|RIP to Wimbledon epics! Isner feels like The Undertaker following final-set tie-break rule|
|Sevilla performance sealed Bernat´s Bayern exit - Hoeness|
|Keita set to miss out as Liverpool monitor injured stars|
|He didn´t want to be here – Guardiola doubts Sancho will return to Manchester City|
|Sarri: Manchester United are the best in the Premier League|
|Mourinho salutes ´brave´ Shaw for ´best possible´ response to criticism|
|No Ballon d´Or distractions for Griezmann, insists Simeone|
|Wigan stand by Hardaker as full-back prepares to undergo rehabilitation|
|Barcelona deny Neymar talk|
|Hazard can win the Ballon d´Or as a Chelsea player, insists Sarri|
|De Bruyne in contention to start in Manchester City´s clash with Burnley|
|Neymar needs to mature to reach Messi levels - Xavi|
|We have a hell of a lot to do - Paine slams Australia batting|
|Kerber and Osaka to meet at WTA Finals|
|Lorenzo out of Japanese MotoGP|
|Real Madrid handed Africa trip in Copa del Rey|
|Allegri: Ronaldo has sharpened Juventus focus|
|No Wade in Wasps squad amid NFL reports|
|Rooney: Five of us controlled Man United dressing room|
|Bayern bosses launch scathing attack on ´disrespectful´ media|
|Saints lock in Lawes to new deal|
|Adelaide United 1 Sydney 1: Debutant Le Fondre rescues a point|
|Madrid can´t look back, only forward - Lopetegui confident of revival|
|Wimbledon to introduce final-set tie-breaks|
|Abbas inspires Pakistan to series victory|
|Madrid boss Lopetegui hopeful on Bale availability|
|Mood is positive at Bayern, insists under-pressure Kovac|
|LeBron offers food for thought after losing Lakers debut|
|Bolt future not an immediate concern for Mariners|
|Hazard: I could finish my career at Chelsea|
|Buffon hoping to avoid Champions League final against Juventus|
|Wilshere not ready for West Ham return|
|Chelsea v Manchester United: Hazard better under Sarri than he ever was with Mourinho|
|The big Friday Premier League quiz|
|Khawaja suffers meniscal tear, doubt for India series|
|Babbel: Klopp must win trophies now at Liverpool|
|LeBron makes spectacular start on Lakers debut, but LA fall to Trail Blazers|
|Broncos thrash Cardinals to end losing streak|
|Price leads Red Sox past Astros and into World Series|
|LeBron makes fast start on Lakers debut with two massive dunks|
|Real Salt Lake 4 New England Revolution 1: Hosts boost play-off chances|
|Simmons gets triple-double in win over Bulls|
|´Ambitious´ Maguire wants Champions League football|
|Henry praises ´charismatic leader´ Falcao|
|WATCH: Cardinals WR Fitzgerald tackles former QB Warner on sidelines|
|Spurs right not to make signings – Pochettino|
|Icardi happy at Inter, close to contract renewal|
|This Week in U.S. Sports: Bosa leaves Ohio State while the World Series is almost set|
|Liverpool boost as Salah and Van Dijk join training|
|Cowboys discuss road woes, Prescott says team ´addressed the elephant in the room´|
|Isner scrapes through in Stockholm, Monfils eliminates Tsonga|
|Hinch says Altuve would be out with injury in regular season|
|WATCH: Schwarzenegger tells LeBron to make Lakers great again|
|LeBron tells Lakers coach Walton: I´m always fresh!|
|Mayweather should come back to fight me - Alvarez|
|Goerges battles past Vekic, Kasatkina wins all-Russian tie|
|Ask 32,000 Gibraltarians if the Nations League is meaningless - Styche|
|They´re going to speak about it for years - Azhar reflects on comical run out|