Rafael Nadal retires from his opening match at the Miami Open

Perhaps they ought to start giving flu shots in the Miami Open locker room.

A day after Roger Federer withdrew from the tournament with a stomach virus, Rafael Nadal retired from his match Saturday, feeling weak, trailing 94th-ranked Damir Dzumhur 2-6, 6-4, 3-0.

Nadal, a four-time finalist here, said he felt “perfect” before the match but started getting dizzy and faint late in the first set. He called for medical help after losing the first two games of the final set, and a doctor took his blood pressure. He returned to the court but lost the next game and called it quits.

Temperatures were in the mid-80s on Saturday, and the humidity made it feel warmer.

“Everything was fine until the end of the first set,’’ said Nadal, who was seeded fifth. “I started to feel not very good. It was getting worse and worse. I tried to resist, but I got a little bit scared of being too dizzy and feeling faint, so I called the doctor, but I felt I was not safe out there. I wanted to finish the match, but I could not.”

Earlier in the day, fourth-seeded Stan Wawrinka lost 6-4, 6-3 to Andrey Kuznetsov, leaving the tournament without three of the top five men’s seeds.

In the night match, two-time Miami Open champion and No. 2 seed Andy Murray had to dig deep to win 6-3, 7-5 over 76th-ranked Denis Istomin of Uzbekistan, who was 1-29 against top-10 foes entering the match.

Nadal hadn’t retired from a match since the 2010 Australian Open quarterfinal — a string of 443 matches. He said it pained him to walk off the court, but he felt he had no choice

“Even with very bad conditions, I have finished a lot of matches in my career because I didn’t want to retire,” he said. “I have felt this way in Australia a couple of times, but [Saturday] was worse. I was struggling for my health, so I decided to stop because I was not sure that I could finish the match that way.

“Hopefully, it is just the extreme conditions combined with the start of a virus.”

Nadal also pulled out of his Sunday doubles match. He and fellow Spaniard Fernando Verdasco were scheduled to play Americans John Isner and Nicholas Monroe.

Dzumhur, a 23-year-old former child actor from Bosnia-Herzegovina, was 0-5 against top-10 players and had dreamed of playing Nadal on stadium court.

But the script Saturday was not what he envisioned.

“Definitely not the way I wanted to finish,” Dzumhur said. “It was really tough to play.

“ I felt also really like tired, heavy, in some moments dizzy on the court. In the first set, we played a very long second game. After that game, I felt like I cannot run that well.”

Dzumhur called for a trainer after the first set.

“I was thinking about retirement, but just said to myself, ‘I’m playing big Rafa. I’m not having a chance to play every day these big guys. I want to enjoy this moment.’ So, I tried to continue. In the end, it was not the way I wanted, but I won.”

His acting skills came in handy after the match. He never let on how excited he was.

“I tried to keep the celebration in me because I didn’t want anybody to see that I’m celebrating when somebody retired,” Dzumhur said. “But inside of me, I felt really proud. I thought about all the fighting that I had through all the years and [Saturday] that I didn’t want to retire, I wanted to fight. That paid off.”

On the women’s side, top-ranked Serena Williams, an eight-time champion in Key Biscayne, said she was unfazed by the weather conditions as she got past Zarina Diyas 7-5, 6-3.

“For me, the biggest adjustment is I usually never use a towel, but here I might have to go grab a towel between points,” Williams said. “I’ve been living here for over 20 years. I actually love these conditions.”

Among the other players who advanced were: Kei Nishikori, who beat qualifier Pierre-Hugues Hebert 6-2, 7-6 (7-4); Milos Raonic, who had 13 aces and got past American Denis Kudla 7-6 (7-4), 6-4; Gael Monfils, Svetlana Kuznetsova, Simona Halep, Jack Sock; and Agnieszka Radwanska, who beat American Madison Brengle 6-3, 6-2.

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