Fourth-seeded Sam Querrey withdrew from the U.S. Men's Clay Court Championship before his first-round match Thursday because of a strained right pectoral muscle.
Querrey, the top ranked American at No. 20 in the world, was about to play wild card Rhyne Williams of the United States when he decided to withdraw.
"When I was warming up today I went hard to test it and if I had one more day or maybe two, I think it would be good," Querrey said. "It's just extreme fatigue in my pec and when I go up to serve and on forehands it catches it and it's achy and painful and I just don't want to tear it."
Querrey won't play again for three weeks and doesn't anticipate an extended layoff.
"Because it's gotten so much better since Monday I think it three days it will be almost 100 percent," Querrey said. "I've got three weeks to get healthy and back on the clay."
Querrey said his next event will be in Madrid.