PARIS (AFP) - World number one 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 quarter-finals.
But it was a tough afternoon for the 34-year-old Williams and her 26th-seeded opponent who needed to sit out a two and a half-hour rain delay before the second set tie-break could commence.
Mladenovic even had a set point at 9/8 in the breaker but finally wilted on a fifth match point.
"She played really, really well and forced me to bring out my best tennis," said Williams, the 2002, 2013 and 2015 champion.
"I feel like I made a tremendous amount of errors but I feel like she kind of forced me to," added Williams.
"In the tie-break, I made it a point to play my game. Up until that point I had not been playing my game. I was playing really defensive. It's not me. So I just wanted to be Serena out there."
Williams saved three break points in the ninth game of the opening set against an aggressive Mladenovic, two with aces.
The American made the 23-year-old Frenchwoman pay in the next game, racing to three set points on her way to the pocketing the opener.
As thunder and lightning rocked Roland Garros, Mladenovic had to save three break points in the fourth game of the second set, two more in the sixth and another in the eighth.
The former junior champion held on until the tiebreak when torrential rain sent the players off Court Philippe Chatrier.
Williams will take a 3-0 lead over Svitolina into her last-16 tie which will have an extra twist as the Ukrainian is being advised by Justine Henin, who was once one of the American's greatest rivals.
"It really doesn't matter. It's just really about going out there and playing your best," shrugged the American.