LONDON - Down a break at 3-1 in the first set, defending champion Novak Djokovic kept his cool on the hottest day in Wimbledon history.

Running off 13 points in a row, Djokovic seized command and stayed in front the rest of the way on Centre Court as he beat Finland's Jarkko Nieminen 6-4, 6-2, 6-3 on Wednesday to reach the third round of Wimbledon on a sweltering day at the All England Club.

With temperatures soaring into the mid-30s C (mid-90s F), former champions Serena Williams and Maria Sharapova and French Open winner Stan Wawrinka also won in straights sets.

Exiting the tournament were fifth-seeded Kei Nishikori, who pulled out with a calf injury before his second-round match, and No. 7 Ana Ivanovic, who fell in straight sets to American qualifier Bethanie Mattek-Sands.

Nieminen started fast, breaking serve in the opening game and going up 3-1. But the top-ranked Djokovic won the next three games, including two at love, and never let Nieminen -- playing in his final Wimbledon before retirement -- back into the match.

"He came out firing some incredible shots," said Djokovic, who also won the title in 2011 and is going for a ninth Grand Slam title. "He made, I think, four winners in the first game, four or five in the second game. He was a break up but I managed to regroup, play some good tennis and get the control on my side."

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Players wrapped ice towels around their necks during changeovers and spectators used umbrellas to block out the sun as temperatures soared to record levels at a tournament known more for its rain delays than summer conditions.

The Met Office, Britain's official weather service, said temperatures reached 35.7 degrees C (96.26 degrees F) at Kew Gardens, the closest observation site to the All England Club.

The previous record was 34.6 C (94.3 F) in the summer of 1976. The Met Office also said it was "the hottest July day on record" in London, with 36.7 C (98.1 F) recorded at Heathrow Airport.

Organizers kept the retractable roof over Centre Court closed in the morning to keep out the heat, then had it partially covering the spectators behind the baselines to give them shade.

Medical officials treated nearly 100 people by mid-afternoon for heat-related issues but no major incidents were reported. A ball boy collapsed during a match on Court 17 and was taken off on a stretcher but was reportedly recovering well after treatment.

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Despite the unusual heat, most players said they weren't particularly bothered.

"It wasn't as bad as I thought," Djokovic said. "People were talking about it and predicting really difficult conditions. But I didn't find it as difficult as I thought it might be."

Djokovic gave the 33-year-old Nieminen a warm embrace at the net and joined the Centre Crowd in applauding for the Finn after his last match at Wimbledon. Nieminen waved to the crowd and kissed his fingers and patted the grass before leaving the court.

"It's his last Wimbledon, so I was just congratulating him on a terrific career," Djokovic said. "He's been around for so many years. A very, very nice guy, one of the nicest guys off the court."

Williams, the top-ranked five-time champion, won her 23rd consecutive Grand Slam match, serving 12 aces in a 6-4, 6-1 win over Timea Babos of Hungary in a late Centre Court match.

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"I hope I'm not playing my best. I have a lot of room to improve," said Williams, who next faces Britain's Heather Watson as the American bids for a fourth straight Grand Slam title and 21st overall.

Wawrinka, seeded No. 4, held serve throughout and beat Victor Estrella Burgos of the Dominican Republic 6-3, 6-4, 7-5.

"Not my best tennis, but typical second round of a Grand Slam," said Wawrinka, who reached the quarterfinals last year.

In one of the day's biggest surprises, 158th-ranked Mattek-Sands upset Ivanovic 6-3, 6-4. Mattek-Sands, who was out for six months last year with a left hip injury, had 32 winners, more than twice the number for Ivanovic, a former No. 1 player who reached the semifinals in 2007.

Fourth-seeded Sharapova, the 2004 champion, beat Dutch qualifier Richel Hogenkamp 6-3, 6-1.

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In one of the tightest matches of the day, No. 9 Marin Cilic -- the 2014 U.S. Open champion -- needed more than 3 1/2 hours to overcome 90th-ranked Ricardas Berankis of Lithuania 6-3, 4-6, 7-6 (6), 4-6, 7-5.

Cilic will next face John Isner, the 17th-seeded American who Matthew Ebden in straight sets.

Seventh-seeded Milos Raonic served 29 aces and held off Germany's Tommy Haas 6-0, 6-2, 6-7 (5), 7-6 (4).