Former champion Thiem bids farewell to US Open after being hammered by Shelton
The Austrian won the U.S. Open in 2020 before suffering a wrist injury in 2021 which kept him out of action for months and saw him slip down the world rankings.
Earlier this year, he announced he would retire at the end of the 2024 season, after playing at his home Vienna Open in October.
"I was struggling enough the last years, especially since I was not able to come back to my level, but once I took the decision this year in March, I was happy about it," Thiem told reporters after losing 6-4 6-2 6-2.
"I was also able to prepare for this new chapter which is coming soon. Honestly, I loved playing on tour, and I was very patient about it, but I also always really enjoyed to be home and to have kind of a normal life at home.
"That's why I think it's not that difficult for me, and that's why I'm really also happy with my decision."
Asked if he had been affected worse by the injury or the psychological toll of being sidelined for a long period, Thiem said: "The basic reason why I'm here now, retiring pretty young is the bad luck with the wrist injury.
"But again, I'm really happy with the career I had before. I never expected that it's going to be that successful, so I don't have really any regrets, and I'm good with that."
The former world number three also reflected on his successes at Flushing Meadows, where he reached the fourth round on his debut and played memorable five-setters against Juan Martín del Potro and Rafa Nadal.
"When I came first here as a pro, I straightaway played fourth round, and that gave me a boost for all the upcoming years," Thiem said.
"If you have really good memories or if you have a good start, it really gives you positive vibes for the future. But still, 2020 is above everything and is also above everything else in my tennis career."