American player Jack Sock celebrates taking the second set against Croatian Marin Cilic, during day three of the ATP World Tour Finals being played at the O2 Arena in London, Tuesday Nov. 14, 2017. ( ... - Easy Branches

American player Jack Sock celebrates taking the second set against Croatian Marin Cilic, during day three of the ATP World Tour Finals being played at the O2 Arena in London, Tuesday Nov. 14, 2017. ( ...

American player Jack Sock celebrates taking the second set against Croatian Marin Cilic, during day three of the ATP World Tour Finals being played at the O2 Arena in London, Tuesday Nov. 14, 2017. ( John Walton/PA via AP)