His coach urged him not to risk potentially damaging his sore ankle further. Fritz stubbornly refused to bow out.
His faith in himself paid off. Fritz upset Nadal 6-3, 7-6 (5) Sunday to win the BNP Paribas Open and snap the 21-time major champion's 20-match winning streak this year.
"It was a complete nonissue, didn't feel it at all, didn't hinder me at all," said Fritz, who tweaked his ankle late in his semifinal win over No. 7 seed Andrey Rublev. Blue medical tape could be seen above his high sock and he had it numbed before the final.
Nadal had his own health issue. He came into the match bothered by painful breathing that he first experienced in a three-set semifinal win. He took two medical timeouts during the final. The first one came after he lost the first set. Nadal went inside with a trainer after tapping his upper left chest. He received treatment on court after falling behind 5-4 in the second set.
"When I try to breathe, it's painful and it's very uncomfortable. It's like a needle all the time inside. I get dizzy a little bit because it's painful. It's a kind of pain that limit me a lot," said Nadal, who turns 36 in June.
"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."
"Of course, the last two months have been amazing, unforgettable, very emotional," Nadal said. "I enjoy things that I never thought I could live again a few months ago."
Fritz didn't waste any time in attacking Nadal, racing to a 5-1 lead in the first set with two breaks. Nadal got a break back in closing to 5-3, but his backhand error gave Fritz another break and the set.
"It's an honor to even be on the same court as this guy," Fritz told the crowd. "I grew up watching this guy win everything."
Nadal fought off a match point on his serve with a forehand winner to tie the second set 5-all. He had two break points on Fritz's serve in the next game, but the American held for a 6-5 lead. Nadal held to force the tiebreaker.
Nadal sent two straight forehands well wide to set up Fritz's second match point. Another Nadal error sent Fritz to the biggest victory of his career not far from where he grew up near San Diego. Fritz's parents, Guy Fritz and Kathy May, are former tour players.
"He told me that I was going to win this tournament one day when I was a little kid," Fritz said. "It was pretty tough not being emotional with my parents, especially my dad. He was just really, really proud of me. It's really tough to get a compliment out of him."
Fritz dropped his racket and collapsed on his back after clinching the title on his second match point. He got up smiling with a look of disbelief on his face. He became the first American man to win Indian Wells since Andre Agassi in 2001.
"I just kept telling myself there's no reason why I can't win this," Fritz said.
Fritz is projected to rise from No. 20 to a career-high 13th in Monday's ATP Tour rankings and will become the top-ranked American player. Nadal moves up to No. 3.
Nadal had already decided to skip next week's tournament in Miami and head home to Spain to prepare for the French Open, which begins May 22.
Fritz is scheduled for a MRI on his ankle Monday and isn't sure whether he'll be able to play in Miami.
The Miami Open begins on Monday. Daniil Medvedev is among the notables who are playing on the men's side, as well as Alexander Zverev, Stefanos Tsitsipas and Matteo Berrettini. Hubert Hurkacz won the event last year.
The Associated Press contributed to this report.