John Isner, the top-ranked American, withdrew from the Australian Open because of a bruised knee. He reached the third round at last year’s Open.
He is ranked #14 after winning two titles in 2012. The only ranked American male left is Sam Querrey, who is ranked #22.
Isner injured his knee at the Hopman Cup last week and withdrew from the international mixed team competition.
"I have been feeling some discomfort in my knee and have recently learned that I have a bone bruise," Isner said in a statement released by Australian Open organizers on Thursday. "Doctors have told me that continuing to play on the knee could result in a more serious injury."
There has not been a dominant American tennis player since the days of Pete Sampras and Andre Agassi. Andy Roddick came close when he won the 2003 US Open but has struggled in the final rounds of major tournaments.
Isner has only made it to one quarterfinal in his career. He is well known for playing the longest match in tennis history at Wimbledon in 2011. It went three days and lasted 11 hours, 5 minutes.
Hopefully this is not a sign of what is to come this year for American men in tennis. The U.S. needs more men in tennis, not fewer.