World No. 1 Serena Williams admitted on Saturday that the health dangers posed by the Zika virus in Rio ahead of the Olympics are a serious concern.

"It's something that's been on my mind. I'm really just gonna have to go super protected," said the 34-year-old American, the defending Olympic champion.

An open letter addressed to the World Health Organisation (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 the 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.

However, the 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 withdrawals although they were 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.

That group could 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.

Meanwhile, defending Roland Garros champion Williams needed five match points before downing French hope Kristina Mladenovic 6-4, 7-6 (12/10) to reach the last 16 on Saturday.

The top seeded American, chasing a fourth French Open title and record-equalling Open era 22nd Grand Slam crown, goes on to face Ukraine's 18th seed Elina Svitolina for a place in the quarterfinals.