Juan Martin Del Potro has withdrawn from the Cincinnati Masters because of fatigue, tournament organisers announced on Monday.
The world number six was beaten by Andy Murray in the final of the Montreal Masters on Sunday, having also won in Washington the previous week.
Argentine Del Potro will be replaced in the draw by Julien Benneteau of France.
On court, meanwhile, Ivo Karlovic served up 21 aces as he upset 13th seed Gael Monfils 6-4 6-7 7-6 in the first round.
After taking the opening set, the giant Croat, who beat Roger Federer on his way to the semi-finals last year, also led 5-4 in the second-set tiebreak.
However, Karlovic's serve then suffered a rare misfire as Monfils reeled off three superb points in a row to force a decider.
The Frenchman also saved three break points in the fourth game of the third set but Karlovic played a superior tiebreak to set up a meeting with another Frenchman, Paul-Henri Mathieu, who beat Germany's Mischa Zverev 6-7 7-5 6-3.
Fifteenth seed Tommy Robredo also fell by the wayside, the Spaniard looking completely out of sorts as he lost to steadily improving Frenchman Jeremy Chardy 6-3 7-5.
Ninth seed Gilles Simon of France eased to a 6-3 6-2 win over American wildcard Wayne Odesnik and will now play Russian Igor Andreev, who beat Nicolas Kiefer of Germany 6-1 7-5.
Croatia's 14th seed Marin Cilic beat former world number one Juan Carlos Ferrero 6-3 6-4 to set up a tie with fellow Spaniard David Ferrer after he in turn beat Switzerland's Stanislas Wawrinka 7-5 6-2.
Jose Acasuso of Argentina set up a second-round meeting with world number one Federer by beating Lukasz Kubot of Poland 6-4 6-3.
Marat Safin, who is set to retire at the end of the season, eased past American Robby Ginepri 7-5 7-6, while Ivan Ljubicic, Benjamin Becker and Nicolas Almagro also advanced.