Serena Williams pulls out minutes before scheduled clash with Marion Bartoli . World No . 1 says back problem has troubled her for the past fortnight . - Novak Djokovic wins his first title of 2011 in Madrid after beating Roger Federer to win 6-3, 7-6 (7-4) 6-4.
Serena Williams pulls out minutes before scheduled clash with Marion Bartoli . World No . 1 says back problem has troubled her for the past fortnight . American targets next month 's Sony Ericsson Open in Miami for comeback .