Mardy Fish has made a winning return to tennis following his operation to treat a frightening heart condition, but immediately felt unwell after coming off court.

The world number 12 beat Spain’s Ruben Ramirez-Hidalgo 7-6 (7/3) 7-5 7-6 (7/1) in a first round clash at Wimbledon and looked comfortable moving around the court.

But afterwards, the US number two felt too ill to attend an obligatory news conference.

“He is feeling unwell,” said an ATP tour spokesman, who added it was “nothing worrying” and Fish had pledged to return on Wednesday, his day off, to speak to media.

Fish had postponed his news conference for half an hour to see if he felt any better but, in the end, decided to cancel.

Tuesday’s match was his first tournament appearance since April, when, as the top seed, he fell at the first hurdle in Houston.

He had a procedure called cardiac catheter ablation in Los Angeles on May 23 to correct faulty wiring in his heart.

Fish 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 draw after world number 10 John Isner was knocked out on Monday.

During his two-hour and 37-minute match on the Court 12 show court against Ramirez-Hidalgo, the world number 82, Fish sent down 24 aces and hit 61 winners.

He was at ease in the third set tie-break, sealing his victory with a cross-court return of serve for his first win since Miami in March.

Fish plays either British wildcard James Ward, or another Spaniard, world number 36 Pablo Andujar, in the second round.