The American beat Ruben Ramirez Hidalgo on Tuesday to set up a second-round clash with Britain's James Ward, an impressive turnaround considering that he was forced to pull out of the French Open last month after undergoing a heart procedure.
The 30-year-old had not played a competitive match in the last two and a half months after taking time off with an accelerated heartbeat.
"I remember a week and a half before the French Open I was just thinking to myself, 'I'm not even close right now', I'm still sleeping with this heart rate monitor," he said on Wednesday in a press conference that had to be rearranged after he felt ill following his first-round victory.
"I hadn't done the procedure yet. I didn't even know that that was necessarily an option quite yet."
After losing a match in Florida in March, Fish visited a doctor suffering with a racing heart beat and he pulled out of a Davis Cup quarter-final against France the following week.
Having made his comeback on Tuesday, he is now trying to play his way back to his best.
"It's just the confidence part," he said. "It's really the only thing.
"It's just me convincing myself that everything is fine and that the doctors have given me the go‑ahead on everything.
"During the day I don't have any issues. Like I said, (it is) just when I don't feel perfect and when I don't feel exactly the way that I feel like I should be, sometimes that's when I get into a little bit of trouble. Over time I'll feel better.
"It's getting better and better. Hopefully I'll be back to normal in no time."
The world number 12 will play Ward on Thursday.
The Briton is currently ranked 173rd and lost his only match against the American in 2010.
"These guys are in the second round for a reason," Fish said.
"He beat a really good player yesterday. He'll be tough for sure."
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