` It 's only the second time I have pulled out from a tournament , and I have never pulled out during a match . Roger Federer withdraws from Qatar Open with back injury . Federer, who had won the title in Doha last year, withdrew on Sunday after feeling pain during his second-round match against Juan Martin Del Potro of Argentina. Top seed Rafael Nadal lost in straight sets to France 's Gael Monfils in second semifinal . Andy Murray sweeps into semifinals of Brisbane International .
Roger Federer withdraws from Qatar Open with back injury . Federer, who had won the title in Doha last year, withdrew on Sunday after feeling pain during his second-round match against Juan Martin Del Potro of Argentina. Top seed Rafael Nadal lost in straight sets to France 's Gael Monfils in second semifinal . Andy Murray sweeps into semifinals of Brisbane International .