Defending champion Marin Cilic benefited from the retirement of Florian Mayer to reach the second round of the Swiss Indoors Basel on Tuesday, and he was joined in progressing by seven-time winner Roger Federer.
Cilic capitalised on the rare absence of Federer here to beat Kei Nishikori in last year's final and he won the first set of his title defence 6-3 at St. Jakobshalle before Mayer succumbed to injury.
The Croatian is likely to face tougher challenges in his bid to retain the crown, though, with Federer in particularly ominous form as he dispatched teenaged American Frances Tiafoe 6-1 6-3.
The evergreen 36-year-old is hunting a seventh title of what has been a memorable renaissance in 2017, with Australian Open and Wimbledon triumphs lifting him to second in the world behind friend and long-term rival Rafael Nadal.
Earlier in the day, Argentine Leonardo Mayer caused a minor upset by ousting eighth-seeded German Mischa Zverev 7-5 7-5.
Jack Sock had to come from behind to avoid a similar fate, the fifth seed ultimately overcoming Canadian lucky loser Vasek Pospisil 3-6 7-6 (9-7) 7-5.
Third seed David Goffin enjoyed a more routine passage through to the second round, the Belgian beating qualifier Peter Gojowczyk 6-2 7-5.
In other matches, Ryan Harrison dispatched Alexandr Dolgopolov 6-4 6-1 and Benoit Paire saw off Steve Johnson 6-3 7-6 (7-4) to set up a clash with Federer.