Emma Raducanu revealed she struggled with her breathing and felt dizzy during her fourth-round Wimbledon clash with Ajla Tomljanovic before being advised to retire.
"I think the whole experience caught up with me," said the 18-year-old who announced herself as a new star of British tennis by reaching the last 16 on her debut at the All England Club.
She was unable to complete her match against the Australian on Monday evening, withdrawing after an off-court medical timeout while trailing 6-4 3-0.
Writing on Twitter, Raducanu said: "Hi guys, I wanted to let everyone know that I'm feeling much better this morning. First up I want to congratulate Ajla on an incredible performance and I'm sorry our match ended the way it did.
"I was playing the best tennis of my life in front of an amazing crowd this week and I think the whole experience caught up with me. At the end of the first set, after some super intense rallies, I started to breathe heavily and felt dizzy.
"The medical team advised me not to continue and, although it felt like the hardest thing in the world not to be able to finish my Wimbledon on the court, I was not well enough to carry on.
"I want to thank the people who have cheered me on every single match, I wanted to win so badly for you! I also want to thank the All England Club, my team, the LTA, my parents and friends.
"Last night will go a long way to helping me learn what it takes to perform at the top. I will cherish everything we have achieved together this week and come back stronger! Can't wait to see what's next on my journey."
Difficulty breathing was given as the official reason for her withdrawal but the initial lack of explanation for what caused it led to intense speculation.
John McEnroe was heavily criticised after jumping to the conclusion immediately after the match on the BBC that she could not handle the occasion.
Andy Murray weighed in, replying to a tweet from Kevin Pietersen about mental toughness being what separated good athletes from great ones, although the former England cricketer later insisted he had not been referring to Raducanu.
Murray wrote: "No question mental toughness can be what separates the best in sport but surely both of you aren't judging her mental toughness on yesterday's match?!
"I think some of what he (McEnroe) said was fair yes ... however the timing of it was a bit off considering nobody had any clue what her issue was injury/illness/breathings issues etc at the time of his comments."
Wimbledon organisers earlier issued a statement wishing Raducanu well.
"We were very sad to see Emma forced to withdraw from her match last night and wish her all the best with her recovery.
"She should be commended for the poise and maturity she has shown throughout the Wimbledon fortnight and we very much look forward to welcoming her back to Wimbledon next year and in the years to come."