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

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

Croatian Marin Cilic celebrates taking the opening set against American player Jack Sock, 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)