ROGER Federer, refreshed after getting a free ride into the U.S. Open quarter-finals, will be back on court at Flushing Meadows on Wednesday to play for a spot in the last four.The Swiss master was gifted a place in the quarters when American Mardy Fish withdrew from their fourth round match because of health concerns but faces a tough challenge from Czech Tomas Berdych.Federer has beaten Berdych in 11 of their previous 15 meetings and has yet to drop a set in this year’s tournament as he chases his sixth U.S. Open title.The winner will play either Olympic champion Andy Murray or big-serving Croatian Marin Cilic, who are also slated to clash on a beefed-up schedule caused by four long rain delays on Tuesday.Murray has won all his matches in straight sets and got progressively better with each appearance."I hope I’m going to play better for the rest of the tournament but you never know,” he said."Conditions change, the opponents (change). You just need to make sure you’re ready for whatever your opponent is going to bring and try and get the win.”Only two of Tuesday’s six singles matches were completed before the foul weather intervened, forcing tournament officials to carry them over until Wednesday in the hope they can finish the last grand slam of the year on time.Serbia’s Novak Djokovic, the men’s champion, was leading Stanislas Wawrinka 2-0 in the fourth round while 2003 winner Andy Roddick and Juan Martin Del Potro, the 2009 U.S. Open champion, had played one point of their first set tiebreak and Janko Tipsarevic was 5-2 up against Philipp Kohlschreiber 5-2.In the women’s quarter-finals, Marion Bartoli grabbed an early 4-0 lead against Maria Sharapova, one of six U.S. Open champions in action on Wednesday.Serena Williams, riding high after winning Wimbledon and the London Olympics gold medal, takes on Serbia’s former world number one Ana Ivanovic.Williams won the last of her three U.S. Open titles in 2008 but is the hot favourite this time."I think I hit unbelievable serves at Wimbledon and the Olympics,” Williams said. "I’m not in that level yet, but I always try to play better in my last matches of a Grand Slam.”The winner will play the victor of the last women’s quarter-final between Italian friends and doubles partners Sara Errani and Roberta Vinci.