Ohio, USA (AFP) — Olympic champion Andy Murray was sent crashing out of the Cincinnati Masters on Thursday, going down to a shock 6-4, 6-4 defeat at the hands of French lucky loser Jeremy Chardy.

Murray, the defending champion at this key US Open warm-up event, won just one match in Toronto last week before withdrawing with a knee injury but he looked impressive in his Cincinnati opener against Sam Querrey on Wednesday.

However, he was distinctly undercooked against Chardy and his 16th unforced error of the second set sent the Frenchman into the last eight.

Four-time champion and top seed Roger Federer turned in another flawless performance to ease past Australian teenager Bernard Tomic, 6-2, 6-4, unleashing 27 winners.

The 2009 US Open winner Juan Martin del Potro, who missed 2010 due to right wrist surgery, had troubles with his left wrist, requiring taping on his arm in his 7-6 (7/2), 2-6, 6-1 defeat of Serb Viktor Troicki.

American 10th-seed Mardy Fish continued to perform well on his home cement courts, beating Czech Radek Stepanek, 6-3, 6-3, with seven aces and 23 winners in 90 minutes and will next play Federer, whom he he has lost to in seven of eight meetings.

Meanwhile, the recent exertions of winning Wimbledon and a pair of Olympic gold medals appeared to be catching up with Serena Williams as the fatigued second seed stumbled through into the WTA quarter-finals.

Williams, 30, scratched her way to a 6-4, 6-3 defeat of Urszula Radwanska. The Polish challenger, younger sister of top-seed Agnieszka Radwanska, led twice in the opening set but was unable to overpower the American.

Williams, who lost serve four times while breaking her opponent on six occasions, said she is not at 100 per cent.

Top-seed Agnieszka Radwanska beat American Sloane Stephens 6-1, 4-6, 6-4 while US Open holder Samantha Stosur beat Elena Makarova, 6-1, 6-3.

Fourth seed, and former Wimbledon winner, Petra Kvitova advanced over China’s Peng Shuai, 6-2, 6-2.