David Goffin's withdrawal due to injury afforded Roger Federer a walkover route into his eighth Cincinnati Masters final, with Novak Djokovic awaiting him in Ohio on Sunday.
Federer, who has won all seven of the finals he has appeared in at the ATP Masters 1000 event, had taken the opening set 7-6 (7-3) and it was 1-1 in the second when 11th seed Goffin was forced to call a halt to the contest.
"David would never pull the plug unless he had to," Federer said in an on-court interview.
"I have a lot of respect for him [but] I'm excited to be in another final in Cincinnati."
Of Djokovic, Federer said: "He's been one of the toughest guys to play against in my career, no doubt about it. He's a great champion. We've had great battles over the years, including here."
The Serbian is a five-time losing finalist in Cincinnati, Federer getting the better of him in the decider in 2009, 2012 and 2015.
A tight, high-quality encounter between Federer and the world number 11 on Saturday initially flew by without a break-point opportunity being offered by either player until the penultimate game, when Goffin had to fight hard to stave off three break and set points.
Federer netted a backhand volley to give up a mini break right from the off in the resulting tie-break, but Goffin promptly gave it straight back by rushing a forehand and dragging it into the net.
A wide forehand off the next point saw the Belgian's situation worsen, and Federer took advantage, sending down his eighth ace of the set to take control, before another netted forehand rally ball saw Goffin succumb.
The 27-year-old, who had been serving below his top speed throughout the opener, then called for the trainer, with his shoulder and right arm both receiving medical attention.
And Goffin lasted just two games of the second set before he was forced to retire.
Djokovic, meanwhile, outlasted Marin Cilic to reach the final, where he will bid to complete the set of all nine Masters 1000 titles.
The Serb saw off the 2014 US Open champion 6-4, 3-6, 6-3 to book another match against Federer, with the pair having met 45 times previously. Djokovic holds a narrow advantage in the head-to-head, with 23 wins to the Swiss' 22.