Rafael Nadal says he suffered from breathing difficulties during his straight sets defeat by Taylor Fritz in the Indian Wells final.
Nadal, 35, appeared out of sorts and quickly found himself 4-0 down before leaving the court for treatment.
He looked more comfortable on his return but struggled to break down a determined Fritz who won 6-3 7-6 (7-5).
“It’s tough for me to breathe. When I try to breathe, it’s painful and it’s very uncomfortable,” said Nadal.
“When I’m breathing, when I’m moving it’s like a needle all the time inside here. I get dizzy a little bit because it’s painful. It’s a kind of pain that limits me a lot. It’s not only about pain, I don’t feel very well because it affects my breathing.”
Spaniard Nadal, who claimed victory at the Australian Open earlier this year, had won his previous 20 matches in 2022.
But he made 34 unforced errors in the match compared to 22 from Fritz.
“Honestly I am sad because of the way I was not able to compete,” he added. “The thing that worries me now, it’s about what’s going on there, what I have to do now to recover and how long it’s going to take.”
Fritz’s win meant he became the first American to win at Indian Wells since Andre Agassi in 2001.
He saved two break points at 5-5 in the second set before winning a tie-break to secure his second ATP Tour title – and first Masters 1000 victory.
There were concerns about his fitness after he suffered an ankle injury in the semi-final win against Andrey Rublev but he showed no ill-effects during his win.
“I can’t even begin to describe how ridiculous it is to play, how I could play today, I’ve never experienced worse pain like I did before the match,” said 24-year-old Fritz.
“I took a couple of steps and screamed. I was trying to act tough and we did a lot of work leading up to the match.
“I was really upset, basically almost crying because I thought I was going to have to pull out.
“I went through a rollercoaster of emotion and all of a sudden being really happy thinking maybe I can play, but it didn’t affect me at all.
“This is one of those childhood dreams you never even think will come true.”
Analysis by BBC tennis correspondent Russell Fuller
Rafael Nadal confessed to being both sad and worried as his 2022 unbeaten run came to an end.
The sadness stems from not being able to complete a perfect hard court season; the worry from just not knowing how this as yet undiagnosed injury may affect his clay court season.
But what a performance from Taylor Fritz after fearing he would not even be able to take to the court.
His blistering start to the match was perhaps a sign of a man who did not know how long his ankle would last. But he soon realised he was in much better shape than he could possibly have imagined and was able to clinch the title in a very tight and tense second set tiebreak.
This tall and powerful former junior world number 1 is rapidly heading into the world’s top 10 – which is his next stated goal.