World number two Murray, who smashed a frame in anger in the final game, said the proximity of next week's Davis Cup final on a claycourt in Ghent had not been a factor in his defeat.
Roger Federer's enduring brilliance and his sheer hatred of losing allowed him to withstand a stunning Kei Nishikori fightback and remain undefeated at the ATP World Tour Finals on Thursday.
Photos from the ATP World Tour Finals matches played in London on Tuesday.
Novak Djokovic handed Kei Nishikori a brutal lesson in hardcourt tennis as the Serb began his bid for a record fourth successive title at the ATP World Tour Finals with a 6-1, 6-1 win on Sunday.