Rafael Nadal retired from a match for the first time in six years after feeling "dizzy" in stifling conditions at the Miami Open.
The Spaniard, seeded fifth, had called for the doctor twice before he pulled out trailing 3-0 in the deciding set against world number 94 Damir Dzumhur.
"Hopefully it's nothing, it's just the extreme conditions," said Nadal.
"I called the doctor a couple of times but I felt I was not safe there, so I decided to go."
Dzumhur, the world number 94 from Bosnia-Herzegovina, was leading 2-6 6-4 3-0 after one hour and 50 minutes when Nadal called it a day.
"Everything was fine until the end of the first set," added the 29-year-old.
"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."
The last match Nadal retired from was the 2010 Australian Open quarter-final against Britain's Andy Murray.
Fourth seed Stan Wawrinka was another leading name to make an early exit, the Swiss player going down 6-4 6-3 to Russia's Andrey Kuznetsov.