The Serbian will take on Juan Martin del Potro on Sunday for a place in the final at London's O2 Arena.
"The fact is that it being the last tournament of the year here in London for most of the players, it is a challenge finding that last drop of energy, physical, mental, to find their best game and go as far as they can," BBC Sport quotes him as saying.
"So that's in my case, same thing. I played a lot of matches this year but for the first time I won all three in my group.
"This is very encouraging for me, so I will try to focus on the semi-final now."