Serena Williams said on Saturday the health dangers posed by the Zika virus in Rio ahead of the Olympics are a serious concern, while Novak Djokovic pleaded for the impact on Brazilians not to be underestimated.

"It's something that's been on my mind. I'm really just gonna have to go super protected," said Williams, the defending Olympic champion, after making the last-16 of the French Open with a 6-4, 7-6 (12/10) win over home hope Kristina Mladenovic.

An open letter addressed to the World Health Organization (WHO) signed by 150 international doctors, scientists and researchers on Friday called for the August Games to be moved or delayed to help prevent the spread of Zika.

Holding the Games in Rio, the second worst affected city in Brazil, would be "irresponsible" and "unethical" and could risk spreading the virus to "poor, as-yet unaffected places" like Africa and South Asia.

However, WHO said moving the Olympics would not have a major impact on the spread of Zika.

The tennis event in Rio has already been hit by no-shows - although not necessarily health-related.

So far, 17th-ranked John Isner of the United States, Austrian 15th seed Dominic Thiem, Bernard Tomic, the world number 22 from Australia and Spanish veteran Feliciano Lopez, 23 in the rankings, have all announced they are giving the Olympics a miss.

They were joined on Saturday by former Roland Garros semifinalist Ernests Gulbis, who panned the sport's appearance at the Olympics as "tennis tourism".

That group of no-shows could still be joined by 2008 champion Rafael Nadal if the Spaniard fails to recover from the wrist tendon injury which caused him to withdraw from the French Open on Friday.

Men's world No 1 Djokovic said any talk of canceling or moving the Olympics was unrealistic.

He also believes the international coverage afforded to the Zika virus ignores the concerns of local people.

"To cancel the Olympic Games is unthinkable, honestly. I mean, many athletes and people already planned in advance," said the Serb after his 6-2, 6-3, 6-3 win over Britain's Aljaz Bedene on Saturday.

"Of course we have to have common sense that health is the most important for anybody that is there.

"But we should not only think about people coming in Rio. How about those people living there, you know? Not talking about them too much. So I think we have to look from different perspectives in order to draw the right conclusion."

The controversial Gulbis, who also made the fourth round in Paris on Saturday, said he had no desire at all to represent Latvia at the Olympics.

"I really don't like that in the Olympic Games there is no points and no prize money. It's a little bit like tennis tourism," he said.

WHO rejects calls to move Olympics

The World Health Organization has ruled out any change in timing or the location of the upcoming Olympic Games in Rio de Janeiro, shunning a call by doctors and scientists to shift the event over the Zika virus.

An open letter addressed to the global health body signed by 150 international doctors, scientists and researchers on Friday called for the August Games to be moved or delayed to help prevent the spread of Zika virus.

Holding the Games in Rio, the second worst affected city in Brazil, would be "irresponsible" and "unethical" and could risk spreading the virus to "poor, as-yet unaffected places" like Africa and South Asia, said the letter.

Zika, which can cause birth defects including a devastating syndrome known as microcephaly in which babies are born with unusually small heads and brains, can be introduced to a new region when a local mosquito picks it up from an infected human.

If it lives long enough, the mosquito then infects people it subsequently bites, starting a vicious cycle.

But WHO said moving the Olympics would not have a major impact on the spread of Zika.

"Based on current assessment, canceling or changing the location of the 2016 Olympics will not significantly alter the international spread of Zika virus," it said in a statement on Friday.

Brazilian authorities on Saturday also said the Games would proceed as planned, with the health ministry saying it would continue to follow the guidance of WHO, which has deemed the risk of Zika infection in August - the middle of winter in Brazil - to be "minimal".

Nearly 1,300 babies have been born in Brazil with irreversible brain damage since the mosquito-borne Zika began to spread there last year.

The virus can also cause adult-onset neurological problems such as Guillain-Barre Syndrome, which can cause paralysis and death.