Serena and Venus Williams moved into the quarterfinals of the Western & Southern Open by handling the afternoon heat, humidity and wind. The conditions were too much for Andy Murray.

Murray's off-day turned into the biggest upset of the tournament, a straight-sets loss Thursday that left him with hardly any hard-court time heading into the U.S. Open.

Murray lost 6-4, 6-4 to France's Jeremy Chardy, an opponent he'd beaten easily the four previous times they'd played. Murray had trouble controlling his shots on a breezy, 85-degree afternoon.

"I was a little bit uncomfortable with those balls bouncing extremely high," said Murray, who left the court as soon as the match ended and headed directly to the interview room. "It was fairly hard to control."

Roger Federer, Novak Djokovic, Mardy Fish and Juan Martin del Potro also reached the quarterfinals Thursday.

On the women's side, Serena Williams won her 19th straight match - her last 12 in straight sets - by beating Urszula Radwanska of Poland 6-4, 6-3. Williams hasn't lost a set since the Wimbledon final, which she won.

She returned to Wimbledon and won the gold medal in singles and in doubles with sister Venus. The travel and the hot conditions have worn her down.

"I have been playing a lot," she said. "Definitely (need) more energy. Been traveling a lot, been training a lot. You know, it's been a really big summer."

Venus Williams beat Sara Errani 6-3, 6-0, a quick match after two long ones the last two days.

- Associated Press