Britain's Jack Draper was forced to retire from his ATP Tour debut after collapsing on court at the Miami Open in searing heat.
Draper called for the trainer midway through the first set and the doctor was called to check his temperature and blood pressure.
But it got worse for the 19-year-old. He was trailing 6-5 on serve against Mikhail Kukushkin before he fell to the ground in the sweltering conditions as he chased a return on set point.
'Obviously a concern' - Djokovic falls on Wimbledon grass worry Wilander
Moments later he sat up with an ice pack on his neck before walking off-court.
"The hard part, he's not used to playing at this sort of physical tempo, this sort of period of time," former British number one Greg Rusedski, who was watching the match, told Amazon Prime.
"It was like an hour and 10 minutes, every point was brutal and it's a step up in class.
"He has to learn from this situation, get the ice towel out from game one. I've got to make sure I hydrate.
"I'm sure he will recover from that. That was the positive from the end of that match."
Later on Thursday Britain's Johanna Konta plays Poland's Magda Linette while Liam Broady takes on Serbia's Miomir Kecmanovic.
Djokovic roars back to beat Britain's Draper in emotional opener
'What a moment' - Draper gets 'special' standing ovation against Djokovic