Mardy Fish says it was tough building up his confidence following his scary heart operation, after feeling the strain on his return to tennis.
The world number 12 beat Spain's Ruben Ramirez-Hidalgo 7-6 (7/3) 7-5 7-6 (7/1) in a Wimbledon first round clash on Tuesday and looked comfortable moving around the court.
But immediately afterwards, the US number two felt too unwell to attend a news conference, suffering with arm ache and a stomach complaint.
Fish said on Wednesday he was feeling "much better".
"I have been taking a lot of pain meds for my arm. The balls are extremely heavy here. I had a pretty bad stomach thing," he added.
"It was probably better for me to try to get home and lie down. It's tough to lie down in the locker room with tons of guys, so that was it. Fortunately it wasn't anything heart-related.
"I'm sure it was just because of the match."
Tuesday's clash was his first tournament appearance since April, when, as the top seed, he fell at the first hurdle in Houston, clearly out of sorts.
He had a procedure called cardiac catheter ablation in Los Angeles on May 23 to correct faulty wiring in his heart.
"It wasn't necessarily if I would ever come back. It was a lot of 'when am I going to come back?'," the 30-year-old said.
"The toughest part is just your confidence.
"The hardest part is just trusting that everything is fine - because it is - and everything structurally is fine.
"It's just me convincing myself that the doctors have given me the go-ahead. The more you keep your mind off it, the better."
The 2011 Wimbledon quarter-finalist said the arrhythmia had made it hard for him to sleep and his heart felt like it was going to burst out of his chest.
Fish is the top American left in the Wimbledon draw.
He faces British wildcard James Ward, who knocked out Spain's world number 36 Pablo Andujar, for a place in the third round in the opening clash on Thursday on the 11,400-seater Court One.