Shelton, 22, was leading Mannarino by two sets to one when he was heard telling father and coach Bryan that “I did something to my shoulder and I don’t know what it is,” describing feeling “in a lot of pain” before calling a mid-game medical timeout.
"I never retired before. I'm not a guy who would retire if I could continue," a dejected Shelton said after the match.
Despite looking visibly compromised on several shots, Shelton played through the fourth set in hope of securing a straightforward victory, but was unable to continue and made the emotional decision to retire upon Mannarino’s return from an off-court break.
Shelton enjoyed a Masters 1000 breakthrough earlier this summer when he won the National Bank Open in Toronto. He followed up that result with a quarterfinal finish at the Cincinnati Open, and won both of his first two rounds in straight sets against Ignacio Buse and Pablo Carreño Busta.
"I've got a lot to be grateful for," said Shelton. "Been pretty fortunate with my life; blessings, talents, a lot of things God's given me. You know, a small setback like this, yeah, it hurts. I was playing really well, I was in form, a lot of confidence. Just so many things to be happy with the way that I was playing, moving on the court, competing.
"But like I said, it's been a great summer, a lot of things to be thankful for. You won't hear me over here pouting about how bad things are with the summer that I've had, the things that I've been able to do in this sport in a short amount of time and the people I got around me, like I said, a lot of blessings."