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Top seed Andy Roddick withdrew from the U.S. Clay Court Championships on Monday because of a lingering hamstring injury.
Roddick first suffered the injury to his left upper hamstring against Andy Murray in the Miami Masters on March 28.
The three-times Houston champion said he tweaked it during his 7-6 6-1 6-4 Davis Cup victory over Spain's Fernando Verdasco in Winston-Salem, North Carolina, on Friday.
"The unfortunate thing about a hamstring injury is that it is one of the most susceptible to re-injury over and over," Roddick said.
"At this point, instead of changing surfaces -- you know how natural my movement is on clay in the first place -- it would probably be best to make sure I get healthy."
Alexander Peya of Austria, the highest-ranked player not to advance from qualifying, will replace Roddick in the draw against Spaniard Albert Montanes on Tuesday.