RIO DE JANEIRO — Dominic Thiem is once again fighting through his matches and enjoying himself, but confidence is admittedly still lacking for the 29-year-old.

Thiem lost in the first round of the Rio Open on Monday, falling to Thiago Monteiro 6-1, 3-6, 7-6 (2).

The former world No. 3 and 2020 US Open champion, who came in ranked at No. 96 after battling right wrist and abdominal injuries, fell to 1-2 on the year. He missed the Australian Open and got his first win last week at Buenos Aires, Argentina.

“I was not convinced enough. It is a so-so leg of confidence. A leg of one match this year,” Thiem said. “Today was a little bit better than I did in Buenos Aires. But still when I step in, it is too many mistakes. I need to find the right timing again, find myself closer at the baseline again.”

Thiem, who won the Rio Open in 2017, dropped the first set against the 83rd-ranked Monteiro in 27 minutes, with the Austrian making a series of unforced errors.

Thiem righted himself in the second set, relying on his forehand and his patience on the baseline. After a back-and-forth third set, the Brazilian won the tiebreaker comfortably to close out the 2-hour, 46-minute match.

“I was never injured before. I didn’t know how long it would take. It is taking as long as it takes. I am not stressing out,” Thiem said. “I have the feeling I was defining myself way too long with results. It is very unhealthy, especially mentally, if I define myself only with results.”

Monteiro will next face either second-seeded Cameron Norrie or Juan Manuel Cerundolo in the clay-court tournament.

Defending champion and second-ranked Carlos Alcaraz faces Mateus Alves in the first round on Tuesday.