Mardy Fish pulled out of his U.S. Open fourth round match with Roger Federer due to unspecified health reasons.
The 30-year-old Fish did miss two months of the season for an accelerated heartbeat and had a medical procedure done in May but it isn't clear if that's what is ailing him, according to reports.
Fish's withdrawal allows Federer to reach the quarterfinal 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 wasted no time advancing to the quarterfinals with a dominant 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 the fourth seed.