World number five Rafael Nadal said his retirement at the Miami Open on Saturday was due to dizziness.
Playing in stifling conditions, Nadal was forced to withdraw from the second-round match as Damir Dzumhur led 2-6 6-4 3-0 in the final set.
Nadal twice required medical attention and, while he attempted to battle through, the 14-time grand slam champion was too scared to play on at the ATP 1000 event.
"Hopefully it's nothing, it's just the extreme conditions," said the fifth-seeded Spaniard.
"I called the doctor a couple of times, but I felt I was not safe there, so I decided to go.
"Everything was fine until the end of the first set.
"I started to feel not very good. It was getting worse and worse and worse. In the second set I realised that I was not able to keep playing. I tried to resist, but I got a little bit scared to be too dizzy.
"I wanted to finish the match, but I seriously couldn't."