Ben Shelton advanced to the last 32 of the Cincinnati Open after just 63 minutes on Sunday, as opponent Camilo Ugo Carabelli was forced to retire due to a knee injury.
Canadian Open champion Shelton was leading 6-3 2-1 and well on his way to victory following a break, when Carabelli jarred his knee on a net approach.
The Argentine took a medical timeout then returned to complete the fourth game of the set, which Shelton served out to love, before calling it quits.
Shelton, who will face Roberto Bautista Agut in the next round, said in his on-court interview: "It's definitely not easy to play a couple of days after winning a title.
"I feel for Camilo and it’s obviously not the way that you want to go through. I hope that he has a quick recovery.
"I'm just excited for the opportunity to play here in Cincinnati. This is the tournament where I had a big breakthrough and one that I never want to miss."
Third seed Alexander Zverev, meanwhile, will face Brandon Nakashima after overcoming another home hope, Nishesh Basavareddy.
The German did not face a single break point and served 12 aces to Basavareddy's one during a 6-3 6-3 success that required just 66 minutes.
Twelfth seed Daniil Medvedev, meanwhile, suffered a 6-7 (0-7) 6-4 6-1 defeat to world number 85 Adam Walton in stifling heat.
The Russian has now won just one match at the Cincinnati Open or Canadian Open in the last two years, having lost to another Australian, Alexei Popyrin, in the second round of the latter event two weeks ago.
Medvedev, who also has just one grand slam match win to his name this year, said of his poor form: "It's very tough in tennis, because I could find 10 reasons and I would not know which is the main reason.
"Whatever we do, we just try to improve. So we sat down with the team and said, 'Where can we do things better? What can we change?' We discussed some things and I'm going to try and implement them."
In the women's singles, Karolina Muchova beat Caroline Garcia 7-6 (7-3) 7-6 (7-0), while Barbora Krejcikova overcame Elina Svitolina in three sets.
Iga Swiatek will not have to play in the third round, advancing straight to the last 16 after Marta Kostyuk withdrew ahead of Monday's matchup due to a right wrist injury.
Data Debrief: Zverev out in front
Zverev's victory over Basavareddy was his 156th at ATP Masters 1000 tournaments.
That means he now has the outright most of any player to be born after the format was introduced in 1990, having surpassed Grigor Dimitrov (155).
