Novak Djokovic cools off during his win over Alejandro Davidovich Fokina Wednesday in Tokyo. Heat has been an issue for tennis players, and Paula Badosa needed withdraw from her quarterfinal match on Wednesday. Patrick Semansky/Associated Prress

TOKYO — With the topic of athletes’ mental health and pressure being discussed at the Olympics, Novak Djokovic said “pressure is a privilege.”

The top-ranked Serb was asked how he deals with the pressure of attempting to accomplish something that no man has done before in tennis: completing a Golden Slam.

Steffi Graf in 1988 is the only tennis player to achieve the Golden Slam, winning all four Grand Slam tournaments and Olympic gold in the same calendar year.

“Pressure is a privilege, my friend,” Djokovic said in answer to a reporter’s question after winning both of his matches at the Ariake Tennis Park on Wednesday, beating Alejandro Davidovich Fokina of Spain to reach the quarterfinals in singles then teaming with partner Nina Stojanovic for another victory in the opening round of mixed doubles.

“Without pressure there is no professional sport,” Djokovic added. “If you are aiming to be at the top of the game you better start learning how to deal with pressure and how to cope with those moments – on the court but also off the court.”

Djokovic, who has already won the Australian Open, French Open and Wimbledon this year and needs the Tokyo Games title and the U.S. Open trophy to complete a Golden Slam, said he has learned to deal with all expectations.

“All that buzz and all that noise is the thing that, I can’t say I don’t see it or I don’t hear it, of course it’s there, but I’ve learned, I’ve developed the mechanism how to deal with it in such a way that it will not impose destruction to me. It will not wear me down,” Djokovic said. “I feel I have enough experience to know myself how to step on the court and play my best tennis.”

Djokovic struggled more with mental pressure at the start of his career when he wasn’t winning as much as longtime rivals Roger Federer and Rafael Nadal. He won the Wimbledon title this month to match Federer and Nadal with his 20th Grand Slam.

“Of course. It’s normal,” Djokovic said. “No one is born with those abilities. Those abilities come with time.”

Tennis player Naomi Osaka and American gymnast Simone Biles are among the Olympians who have talked openly about their mental health troubles.

Djokovic’s only Olympic medal was bronze in singles from his first Olympic appearances at the 2008 Beijing Games. He then lost to Juan Martin del Potro of Argentina in his next two Games: in the 2012 bronze-medal match and the first round at the 2016 Rio de Janeiro Games.

“Rio was (a) very tough loss for me,” Djokovic said. “I came into Rio as also a big favorite, winning four out of last five Grand Slams, No. 1 in the world. So I know that feeling; it’s kind of a similar feeling now. But I’m a more experienced player; I know mentally what I need to do and how to work things around on and off the court in order to feel the best that I possible can feel.”

Daniil Medvedev, pauses during his match against Fabio Fognini on Wednesday to deal with the extreme heat. Patrick Semansky/Associated Press

EXTREME HEAT: Bent over in exhaustion before serving. Resting on his racket between points. Grasping for a rubber tube blowing cool air next to his seat on changeovers. Two medical timeouts and one visit from a trainer.

Daniil Medvedev was struggling so much with the suffocating heat and humidity at the Ariake Tennis Park on Wednesday that at one point the chair umpire, Carlos Ramos, asked him if he could continue playing.

“I can finish the match but I can die,” Medvedev replied. “If I die, are you going to be responsible?”

Afterward, Medvedev said he felt “darkness” in his eyes.

“I didn’t know what to do to feel better,” the ROC player added. “I was ready to just fall down on the court.”

Somehow, the second-seeded Medvedev still pulled out a 6-2, 3-6, 6-2 win over Fabio Fognini of Italy to reach the quarterfinals at the Tokyo Olympics.

Spanish player Paula Badosa was less fortunate. She left the court in a wheelchair after retiring from her quarterfinal match against Marketa Vondrousova because of heatstroke. Badosa also had to withdraw from a mixed doubles match later with partner Pablo Carreno Busta.

Vondrousova, the Czech player who eliminated Naomi Osaka a day earlier, had won the first set 6-3. She’s now in the semifinals and into the medal rounds and will next face fourth-seeded Elina Svitolina of Ukraine. Ninth-seeded Belinda Bencic of Switzerland will play 15th-seeded Elena Rybakina of Kazakhstan in the other semifinal match.

After some rain a day earlier, the temperature rose to 88 degrees but the heat index made it feel like a sizzling 99 degrees. The problems the players faced raised questions over why organizers did not grant requests earlier in the tournament from Medvedev and other players to move all of the tennis matches at the Games to the evening.

As Wednesday’s play neared its conclusion, organizers announced that matches would begin at 3 p.m. starting Thursday to make it easier on the players. Matches had been starting at 11 a.m.