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.
He begins on Tuesday night against Serb Filip Krajinovic.
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 face either Romania's Marius Copil or American Ryan Harrison in the last 16.