A back injury suffered during a Sunday practice session with compatriot Roger Federer forced Stan Wawrinka to pull out of the Swiss Indoors tournament on Monday, while Marin Cilic eased into the second round in Basel.
"I turned my back, and retiring was the only decision I could make," said the three-time Grand Slam champion, adding that he will end his season early and not play at the Paris Masters next week.
The withdrawal of Wawrinka, ranked 68th in the world as he continues to fight to fully recover more than a year after two knee surgeries, comes after Federer made an injury confession of his own.
Federer revealed in the run-up to the start at the St Jakobshalle that he had been suffering since the summer with a hand injury, which the 20-time Grand Slam winner insisted was fine now.
Federer, the holder, is seeking a ninth title at the event where he got his start in the sport as a teenage ballboy.
Wawrinka has been playing in fits and starts this season, losing in the Shanghai first round this month after reaching the semi-finals in September in St Petersburg.
Wawrinka, twice a semi-finalist in Basel, was to have faced Adrian Mannarino in the first round on Wednesday.
Later on Monday, 2016 winner Cilic saw off rising Canadian star Denis Shapovalov in straight sets to reach the second round.
Australian Open runner-up Cilic is one of two former champions in the draw alongside Federer, and he brushed aside teenager Shapovalov 6-4, 6-2.
The world number six will next face Romania's Marius Copil, who defeated American Ryan Harrison 6-2, 7-6 (10/8).
Finnish-born home player Henri Laaksonen won his first match in Basel after five previous first-round losses, beating Roland Garros semi-finalist and sixth seed Marco Cecchinato 6-4, 6-2.
Pranjesh wins
New Delhi: Carrying forward his good form, Prajnesh Gunneswaran made a winning start at the Liuzhou Challenger tournament in China but it was curtains for Ramkumar Ramanathan and Sumit Nagal, who yet again fell at the first hurdle.
Prajnesh, who ended runners-up at the Ningbo Challenger last week, downed Frenchman Johan Tatlot 6-4, 7-5 in his opening round of the $ 50,000 hard court event.
The seventh seed Indian will now clash with Egypt's Mohammed Safwat for a place in the quarterfinals. Prajnesh has beaten Swafwat thrice this season.
However, bad results continue to chase Ramkumar and Nagal.
Ramkumar, who has been struggling after his breakthrough performance at the Hall of Fame Open, where he reached the final for the first time in an ATP250 event, lost his opening round to Spanish qualifier Alejandro Davidovich Fokina 6-7(3), 3-6. Qualifier Nagal lost his first round 2-6, 3-6 to Japanese eighth seed Tatsuma Ito.
In the doubles, Arjun Kadhe and Yaraslav Shyla moved to the quarterfinals with a 6-4, 6-3 win over Liam Broady and Duckhee Lee.
Meanwhile, at the Basel indoors, India's top doubles player Rohan Bopanna and his partner Parcel Granollers eased past Spanish duo of Guillermo Garcia-Lopez and David Marrero 6-1, 6-4.