Friday 8 November 2013 09:32, UK
Novak Djokovic may be fiery and moody on court, but his mental strength is the driving force behind his great run, says Annabel Croft.
Watch every session of the ATP World Tour Finals - which run from November 4-11 - on Sky Sports. Click here for coverage details. You can catch all the action from the ATP World Tour Finals with a Sky Sports Day Pass. For more information click here