With an upset bid of top-seeded Roger Federer awaiting him, unspecified health reasons forced Mardy Fish to pull out of his U.S. Open fourth round match with Federer on Monday, according to reports.
"I don't know," Fish said after his third round win over Gilles Simon about what he'd have to do to beat Federer. "A lot more than I did today."
According to reports, Fish had looked tired in his previous two wins over Simon and Nikolav Davydenko.
Fish's withdrawal allows Federer to reach the quarterfinals of a grand slam for the 34th consecutive time. Federer won five consecutive U.S. Open titles from 2004 to 2008 but has not won one since.
On the women's side, Serena Williams cruised to the quarterfinals of the U.S. Open with a message-sending 6-0, 6-0 win over the Czech Republic's Andrea Hlavackova.
Williams has won three U.S. Open titles in her career but hasn't won since 2008. After her gold medal winning performance at the London Olympics, she came into the tournament as the favorite even though she's only the fourth seed.
For a complete list of Sunday's matches, head over to USOpen.org.