Andy Roddick withdrew from the Rogers Cup on Tuesday because of an undisclosed illness.
The eighth-seeded Roddick will be replaced by France's Paul-Henri Mathieu in the main draw. After losing in straight sets in the third round at Washington last week, Roddick said he "didn't feel right physically."
Roddick dropped to No. 11 in this week's ATP rankings, the first time since the rankings began in 1973 that there is no one from the U.S. in the top 10.
In second-round action Tuesday, Wimbledon runner-up Tomas Berdych ousted Sergiy Stakhovsky 6-2, 6-4 in 90 minutes.
"I know him quite well, what his style of the game is, and it's always tough," Berdych said. "He's playing a lot of slice, chip-and-charge and coming to the net, and sometimes really fast, coming in from the baseline. You don't know what to expect."
Roger Federer, ranked No. 3, is scheduled to face Argentina's Juan Ignacio Chela in the evening session.
Frank Dancevic will also play in the night cap on center court, facing off against Switzerland's Stanislas Wawrinka.
Berdych is making his sixth consecutive appearance at the Rogers Cup. He fell to world No. 1 Rafael Nadal in the Wimbledon final earlier this year.
Berdych, ranked No. 7 on the ATP Tour, has won five singles titles over his career, but none this year.
"I wouldn't change it for anything," he said. "I was really sad that it didn't happen in Wimbledon, because there was really something special, that I'd never been in a Grand Slam final."
In other matches, Serbia's Viktor Troicki upset 11th seed Marin Cilic 6-4, 7-6, while Ukraine's Alexandr Dolgopolov earned a 1-6, 6-2, 7-6 win over Germany's Philipp Petzschner. Italy's Fabio Fognini won 6-2, 6-4 over the Czech Republic's Radek Stepanek in another early match.
Also, Chile's Fernando Gonzalez has withdrawn with a left calf injury and will be replaced by India's Somdev Devvarman.