Serena Williams pulled out just before scheduled clash with Marion Bartoli . World No . 1 says back problem has troubled her for the past fortnight . The 15-time grand slam winner added that , having returned to the pinnacle after two and a half years during which her life and career were threatened following a freak foot injury in 2010 , being No . 1 is no longer her primary goal . ` American targets next week 's Sony Ericsson Open in Miami for comeback .
Serena Williams pulled out just before scheduled clash with Marion Bartoli . World No . 1 says back problem has troubled her for the past fortnight . American targets next week 's Sony Ericsson Open in Miami for comeback .