Former British number one Annabel Court witnessed Djokovic's determination up close from court-side and explained why the Serb is so difficult to beat indoors.
"It is quite astonishing when you are down at court level because he is stretching for balls that not many players could get to and then get it back with a little bit of interest on it," she said.
"His core is so strong, stable and balanced when he is out of position, whereas with a lot of other players when they are out of position they are off balance and they can't get enough weight going through the ball.
"I also think that his sliced backhand, which he has improved over time, is getting better and better. In the past I didn't think technically that it was a very sound shot and it didn't have a lot of real sting on it, but now it is really starting to hug the court and stay very, very low.
"In some of those rallies they were really chipping the ball; they were drawing oohs and aahs from the crowd down there because it was wonderful to watch.
"Djokovic is coming into the net more; there are so few gaps in his game it's really tough to see how players can get past him."
Don't miss day two of the ATP World Tour Finals on Sky Sports. The action begins at 1.30pm on Sky Sports 1 HD, then continues on Sky Sports 3 HD at 6pm.
