Grigor Dimitrov admitted he was still dealing with the effects of coronavirus after testing positive for COVID-19 last month.
The three-time grand slam semi-finalist was one of several players to contract coronavirus at the Adria Tour, with Novak Djokovic among them.
Dimitrov, 29, played at the Ultimate Tennis Showdown (UTS) on Saturday and Sunday, suffering losses to Richard Gasquet and Feliciano Lopez.
MORE: Halep set for Palermo Open as quarantine worries ease
The Bulgarian later revealed he was still feeling the impact of contracting COVID-19.
"I'm just trying to put in effort every single day," he told Tennis Majors.
"Even if I'm tired or not, to keep progressing. One day I feel really well and I have about four hours to be out, but then all of a sudden I need to completely shut down to take a nap or just rest.
"I have to go through that process as much as possible. Hopefully I'm going to recover fully."
Dimitrov, the world number 19, said he was grateful to be playing again after his initial struggle with coronavirus.
"I guess it [COVID-19] is different for everyone. I was not breathing well. I was tired. I had no taste, no smell," he said.
"Everything you could possibly think of. It was no fun. To be honest, I'm lucky to be on the court right now.
"I don't take each day for granted. I really appreciate being here. It's so nice that during time off you can come out and play with your competitors."
There have been more than 16 million confirmed cases of coronavirus worldwide, with the death toll exceeding 650,000.
The ATP Tour season was suspended in March and is scheduled to restart next month.