Cincinnati, Ohio - Roger Federer will begin resuscitating his summer after last week's Canadian crash out, as the opening day of the Cincinnati Masters was hit by a pair of late injury defections on Monday.

The Swiss superstar learned the name of his opening opponent in Tuesday's second round, as US player Robby Ginepri overcame Swedish veteran qualifier Jonas Bjorkman 6-0, 7-6 (7-5).

Federer is looking for a vast improvement in his game with Toronto champion Rafael Nadal breathing down his neck, just 300 points behind in the rankings race after winning his last five tournaments.

Nadal has beaten the world number one this year at the French Open and Wimbledon to establish his credentials as the most serious threat yet to the top ranking that Federer has now held for 235 weeks.

"The most important is trying to win some matches here," said Federer, who lost in the Toronto second round to France's Gilles Simon. "I didn't get that last week, unfortunately, which was just disappointing."

"It's an important time. I'm feeling fit and healthy, which is always key as well, during a very difficult schedule this year for we players."

Federer's Davis Cup teammate Stanislas Wawrinka, the ninth seed, withdrew as a precaution before the August 8 start of the Beijing Games with right knee pain.

Doctors have advised him to rest before travelling to China for the Olympic event, resulting in a pullout from his opening match against Swede Thomas Johansson in the Ohio tournament.

"I have some small pains in the knee, and I don't want to take risks," Wawrinka said. "My goals are the Olympics and the US Open. I'll go home and rest a few days, then begin light training."

Wawrinka said that he felt the problem during his third-round Toronto loss to Andy Murray.

Meanwhile, Croatian Mario Ancic withdrew with illness and was replaced by Chile's Paul Capdeville.

On court, two seeds advanced as number 12 Tommy Robredo ousted luckless Mardy Fish 6-2, 6-2 to put the veteran US player on a five- match losing streak. The 
16th-seeded Croatian Ivo Karlovic beat Brazilian Thomaz Bellucci 6-3, 6-4.

Robredo improved to 3-all against Fish. "Mardy is always a tough opponent, and last time I played him was in Australia, and he destroyed me in three sets," he recalled.

"I had to be very focused from the beginning. It's a great win and a great start."

Czech Tomas Berdych scored the week's first upset with a defeat of number 13 seed Fernando Gonzalez 6-3, 3-6, 6-3. Latvian Ernests Gulbis defeated Finn Jarkko Nieminen 7-6 (9-7), 6-2, while Italy's Andreas Seppi put out Spaniard Feliciano Lopez 7-5, 7-5. (dpa)