Djokovic will play Olympics only if fans allowed
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The world number one's reaction came after calls grew in Japan for the Games to take place behind closed doors, with one doctors' association saying the event should be cancelled altogether.
"If not, I'd think twice about participating."
Roger Federer said athletes needed a firm decision on whether the event is going ahead, adding he was still in two minds.
Japanese stars Naomi Osaka and Kei Nishikori have both raised concerns about whether Tokyo should be hosting the Games at all.
Medical groups have warned the massive event could introduce new variants to Japan even as parts of Japan, including capital Tokyo, are still under states of emergency.
Haruo Ozaki, chairman of the Tokyo Medical Association, said on Thursday that holding the Games without spectators "is the bare minimum given the current situation".
"This is the Olympics in a time of emergency," he said at a press conference.
Organisers have already barred overseas fans, while a decision on domestic spectators is expected in late June.
Tokyo's current virus restrictions allow venues to have up to 5,000 fans or 50 percent capacity, whichever is fewer.
Djokovic was speaking after he booked his place in the semi-finals in the Belgrade tournament with a 6-1, 6-0 win over Federico Coria, a victory that puts him fifth on the list of Open era wins.
The 56-minute demolition of the 96th ranked Argentine took Djokovic to 952 wins, putting him one ahead of Guillermo Vilas.
"It was a great performance," said Djokovic, who is tuning up for the French Open which begins in Paris on Sunday.
"And probably one of the best matches, if not the best match, I played this year. I felt fantastic on the court from the first point."
Djokovic will face either fellow Serb Dusan Lajovic or qualifier Andrej Martin in the semi-finals.
He improved to 18-3 for the season and is attempting to reach his third final of the year.
Djokovic won the Australian Open for a ninth time but lost the Italian Open final on clay to Nadal.
Vilas, also an Argentine, played professionally between 1969 and 1992, reaching eight major finals, winning four.
He was a four-time French Open finalist, winning in 1977 with a straight sets demolition of the American Brian Gottfried.