Rafael Nadal was forced to retire from his second-round match against Damir Dzumhur at the Miami Open on Saturday evening.
The former world number one was trailing 2-6 6-4 3-0 to his Bosnian opponent when he decided to throw in the towel because he felt unwell.
"Everything was fine until the end of the first set. Then I started to feel dizzy, not very good," the 14-time Grand Slam champion told reporters.
"It was getting worse and worse. I wanted to finish the match, but I seriously could not.
"It was tough, because I felt I was playing well. I stopped because I was concerned for my health."
Nadal last quit during a match in the quarter-finals of the Australian Open six years ago against Andy Murray.
