Spain's David Ferrer returns against Argentina's Juan Martin Del Potro during their group B singles match in the round robin stage on the second day of the ATP World Tour Finals tennis tournament in London on November 6, 2012.   Spain's David Ferrer extending his winning run to 11 matches with a hard-fought 6-3, 3-6, 6-4 victory over Juan Martin del Potro at the ATP Tour Finals.   AFP PHOTO / CARL COURT        (Photo credit should read CARL COURT/AFP/Getty Images)