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."
|United States 1 Peru 1: Flores salvages late draw for visitors|
|Martinez not worried about Belgium depth|
|Red Sox move ahead of Astros in ALCS|
|Depay ´a joy to watch´, says Koeman|
|Timberwolves owner looking to trade Butler, backs Thibodeau|
|Shepard says Giants have ´total faith´ in Manning|
|Hazard is out of this world – De Ligt enjoys Belgium test|
|Tite: Brazil were better than Argentina|
|Foley eyes World Cup after re-signing with Wallabies, Waratahs|
|Low slams decisive France penalty call|
|I fined him for his goal at Old Trafford! Giggs delighted for Wales hero Wilson|
|Matuidi lauds Deschamps´ tactical nous after Germany win|
|Scaloni excited by Argentina despite Brazil loss|
|Tsonga back in business, Dzumhur dumped out in Moscow|
|Germany suffer six defeats in a year for first time|
|Many clubs have asked about Piatek - agent|
|Bartomeu: Barca open to defensive reinforcements after Vermaelen injury|
|Bonn spoil PAOK´s landmark European game|
|Konta conquers Mertens in Moscow|
|Republic of Ireland 0 Wales 1: Wilson stunner moves Giggs´ side top|
|France 2 Germany 1: Griezmann delivers Low blow as Les Bleus reign in Paris|
|Belgium 1 Netherlands 1: Groeneveld cancels out Mertens strike|
|Argentina 0 Brazil 1: Miranda heads late winner after rivalry fails to ignite|
|Messi would struggle in this Manchester United team – Scholes|
|Falcons place Devonta Freeman on injured reserve|
|Westbrook ruled out of Thunder opener|
|Wilder´s team open to Joshua talks|
|Brentford appoint Thomas Frank as new head coach|
|England´s perfect performance sets the standard - Trippier|
|Odell Beckham Jr. needs to create headlines on the field, says Giants owner|
|Robshaw joins list of England absentees|
|Chile need goals from Sanchez despite United form - Rueda|
|Modric reveals Ronaldo respect, ´will never play with´ Messi|
|Osaka can be one of the greats, says coach Sascha Bajin|
|Liverpool´s Naby Keita carried off injured in Guinea match|
|Duminy to miss Australia tour for shoulder surgery|
|F1 Raceweek: Ferrari ready to ´challenge the impossible´ as Hamilton nears title|
|Umesh replaces injured Thakur in ODI squad|
|IAAF delay implementation of testosterone ruling amid Semenya challenge|
|Fakhar and Sarfraz lead Pakistan recovery after Lyon blitz|
|Japan see off Uruguay in seven-goal thriller|
|Arda Turan given huge fine by Istanbul Basaksehir after alleged brawl with singer|
|Busquets no better or worse than Dier – does Opta data back up Allardyce claim?|
|United boss Mourinho charged for alleged post-match comments by FA|
|Halep withdraws from Kremlin Cup ahead of WTA Finals|
|Renato Sanches eager for further Bayern Munich minutes|
|Nations League standings: Promotion, relegation and race for finals|
|Windies coach Law banned for ´inappropriate comments´ in India Test|
|Carrick denies Manchester United stars are ´not trying´|
|Ferrari struggling under pressure of chasing Mercedes, says Massa|
|Australia desperately need Smith and Warner back - Warne|
|Draxler warns Germany: Mbappe has 300 hp!|
|Draymond Green says Warriors will have to ´bring it´ every night in the West|
|England´s ´wonderful bravery´ leaves Southgate excited for the future|
|Jayasuriya: I always conduct myself with integrity and transparency|
|Northern Ireland stars hurt by lack of club football, says O´Neill|
|Rodgers inspires late Packers comeback|
|Brewers blank Dodgers to take 2-1 NLCS lead|
|Cavaliers, Nance agree to four-year extension|
|Ramos wants Spain to believe in Luis Enrique´s team|
|De Laurentiis: I´ve spoken to Genoa and agent about Piatek|
|Kaka tips bright future for Paqueta after Milan agreement|
|Suns star Booker set to return for season opener|
|Warriors coach Kerr not expecting fast start from NBA champs|
|Fultz to start for 76ers as Brown eyes NBA Finals|
|Maltese champions Valletta want Usain Bolt|
|It would have been normal to kill them! Luis Enrique slams abysmal Spain|
|Seahawks, Trail Blazers owner Paul Allen dies aged 65|
|Red Sox ace Sale released from hospital, still set for game five|
|Beer-throwing fan banned by Patriots|