Juan Martin del Potro "cried a lot" after his shock win over Novak Djokovic and had less than five hours sleep before his second-round match at the Rio Olympics.
The Argentinian stunned world number one Djokovic in the first round on Sunday, before continuing his run with a win over Joao Sousa a day later.
Del Potro, the 2009 US Open, has seen his career threatened by wrist injuries, only making his competitive return in February this year.
The 27-year-old said he struggled to believe his win over Djokovic, who he also eliminated at London 2012.
"I cried a lot last night. Tears of joy, of happiness, for the many people who supported me through my injury and also at this moment," Del Potro said after his second-round win.
"To be able to give back to them from a court and not from a clinic or a hospital is the best way. I'm enjoying it a lot."
Del Potro battled against Sousa before claiming a 6-3 1-6 6-3 victory in one hour, 58 minutes at the Olympic Tennis Centre.
Having set up a third-round meeting with Japan's Taro Daniel, Del Potro was unhappy with organisers.
"At the end the match it was complicated and my legs were heavy," he said.
"Last night I went to bed at 4:30 (07:30 GMT) in the morning. Today I was having breakfast at nine o'clock (12:00 GMT).
"I still can't believe I beat Djokovic and had to play again so soon. And I can't believe I'll have to play doubles [today]. It is very hard, but we'll do what we can."