Spain's Carlos Alcaraz will join 24-time Grand Slam champion Novak Djokovic in the Olympic men's singles quarter-finals.

But Alcaraz and 14-time French Open champion Rafael Nadal were beaten in the men's doubles by USA's Austin Krajicek and Rajeev Ram - losing 6-2 6-4.

Reigning French Open champion Alcaraz defeated Russian Roman Safiullin, who is competing as an Individual Neutral Athlete, 6-4 6-2.

Earlier, Djokovic beat Germany's Dominik Koepfer 7-5 6-3 in very humid conditions, as the Serb continues his quest for the Olympic men's singles gold that has so far eluded him.

"I'm soaking wet, honestly," Djokovic said moments after his win. "It's a very, humid day [and] I just hope it rains so it cools down the temperature a bit and the air.

"But it is what it is. You have to kind of accept and embrace the conditions and it's the same for you and your opponents, so you have to try to make the best out of it."

French Open finalist Alexander Zverev overcame a spirited start by Australia's Alexei Popyrin, to reach the last eight.

The rest of the quarter-final line-up includes Greece's Stefanos Tsitsipas, Canada's Felix Auger-Aliassime, Italy's Lorenzo Musetti, Norway's Casper Ruud and the USA's Tommy Paul.

French Open women's champion Iga Swiatek went through - having been tested in her quarter-final, she was leading 6-1 2-6 4-1 when after the USA's Danielle Collins retired in the deciding set.

Germany's three-time Grand Slam champion Angelique Kerber's career finally came to an end, after losing to China's Zhang Qinwen 6-7 (4-7) 6-4 7-6 (8-6).

"I've achieved everything I dreamed of," Kerber said, in her best result of the season.

"I was number one, won Grand Slams, won a [silver] medal in Rio, so what more could you want in the end?"

Kerber triumphed at the 2016 Australian and US Opens before claiming the Wimbledon title two years later.