KEY BISCAYNE, Fla. (AP) 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.
''It was getting worse and worse and worse,'' the Spaniard said. ''I get a little bit scared - too dizzy. I felt I was not safe, so I decided to go. I wanted to finish the match, but I seriously couldn't.''
His departure left the draw without three of the five highest-seeded men. No. 4 Stan Wawrinka lost to Andrey Kuznetsov 6-4, 6-3, and No. 3 Roger Federer withdrew Friday because of a stomach virus.
In the night session, No. 2 Andy Murray won his opening match against Denis Istomin, 6-3, 7-5.
Eight-time champion Serena Williams avoided the upset bug and seemed unfazed by the weather. The South Floridian earned her 20th consecutive victory at Key Biscayne and reached the fourth round by beating Zarina Diyas 7-5, 6-3.
''I live down the street, and I'm used to these conditions,'' she said. ''Even though it was a little humid, I'm used to it and I love this weather.''
Nadal's match lasted less than two hours, but the temperature approached 90 degrees and was even higher on the sunbaked stadium hardcourt.
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.''
Nadal later pulled out of doubles.
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 is a 23-year-old Bosnian who was a child actor before he decided to focus on tennis. Ranked 94th, he 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 bothered the 154-pound Dzumhur, who consulted with a trainer following the first set.
''It was really tough to play,'' he said. ''I felt really tired and heavy and in some moments dizzy.''
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).
On the women's side, No. 8 Petra Kvitova lost to No. 30 Ekaterina Makarova 6-4, 6-4. No. 3 Agnieszka Radwanska, the 2012 champion, eliminated Madison Brengle 6-3, 6-2.