Rafael Nadal said he was having trouble breathing direct sets against Taylor Fritz in the Indian Wells final.
Thirty-five-year-old Nadal did not look well and quickly found himself 4: 0 before leaving the court for treatment.
Upon his return, he seemed more comfortable, but had trouble overcoming the solid 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.
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"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."
The Spaniard Nadal, who won the Australian Open this year, has won the last 20 matches in 2022.
But he made 34 unforced mistakes in the game, compared to 22 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."
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Fritz's victory means he will become the first American to win in Indian Wells from Andre Agassi in 2001
He kept two breaks at 5-5 in the second set before winning a tie-break and winning his second ATP Tour title and first Masters 1000 victory.
His health was worried after he was injured in the semi-final victory over Andrei Rublev, but there were no bad consequences for his victory.
"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.
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"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 admitted that he was sad and feared that his undefeated ride in 2022 was over. Sadness comes from not finishing the perfect hard-court season; afraid he doesn't know how this undiagnosed injury could affect his time on clay.
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But what Taylor Fritz looked like after fearing he would no longer be able to stand trial.
His frantic start to the game is probably a sign of someone who doesn't know how long he has been. But he soon realized that he was in better shape than he could have imagined, and he was able to win the title in a very tight and exciting second set tiebreak.
This long and strong former junior world number one is fast reaching the top 10 in the world - this is his next set goal.