KEY BISCAYNE, Fla. -- Rafael Nadal slumped in a changeover chair, the blood pressure gauge strapped to his mighty left arm serving as a scoreboard.
He was on the verge of defeat, and a few points later he retired from a match for the first time in six years.
Nadal faded in the subtropical heat and conceded after falling behind in the third set of his opening match Saturday at the Miami Open against Damir Dzumhur.
The No. 5-seeded Nadal trailed 2-6, 6-4, 3-0 when he called it quits after losing a point to fall behind 30-15. He had earlier consulted with a trainer between games three times, and had his blood pressure checked.
Nadal said he started feeling badly at the end of the first set.
"I start to feel myself not very good," the Spaniard said. "It's getting worse, worse, and worse, so finally in the second set I realize that I was not able to keep playing.
"I tried to resist, but I get a little bit scared to be too dizzy. So I call the doctor a couple of times, but I felt that I was not safe there, so I decided to go. I wanted to finish the match, but I seriously could not."
Nadal's match lasted less than two hours, but the weather was sunny and humid, with a temperature approaching 90 degrees and even higher on the stadium hard court.
Nadal said he felt fine before the match and wasn't sure whether he was contracting an illness.
"Hopefully, it's just the extreme conditions," he said. "It's tough for me, because I felt I was playing well. It's a hard accident. That's life."
The 14-time Grand Slam champion hasn't won a tournament since August. But it had been 443 matches since he retired -- in the 2010 Australian Open quarterfinals against Andy Murray.
Key Biscayne remains one of the biggest tournaments Nadal has never won. He's 0-4 in finals.
Dzumhur, a 23-year-old Bosnian ranked 94th, rallied with a variety of tactics, including half a dozen drop shots for winners and an occasional serve and volley. Nadal had to work hard from the start, failing to convert nine break-point chances during a marathon second game before Dzumhur double-faulted to lose serve.
The heat also appeared to bother the 154-pound Dzumhur, who consulted with a trainer following the first set.
Nadal finished 3-for-13 converting break points. Dzumhur was 4 for 4.
No. 30-seeded Thomaz Bellucci retired from his match after two sets because of dehydration. In other men's play, No. 13 John Isner double-faulted on his final two service points and lost to qualifier Tim Smyczek 6-2, 2-6, 7-6 (5).
In the night session, No. 2 Murray won his opening match against Denis Istomin, 6-3, 7-5, despite a brief disagreement with the chair umpire after Murray felt he mistakenly was given a women's ball to serve with on break point.
"I saw the logo just as I was about to serve on break point down," Murray said afterward. "I was frustrated with that. The ball's the women use are smaller and quicker."
Men and women use the same size balls and each have equal pressure. In most events, including Miami, men use extra-duty felt, which slows the overall speed; women, meanwhile, use regular-duty felt.
Information from The Associated Press was used in this report.