Novak Djokovic booked his place in the semifinals of the ATP Tour Finals with an emotional 7-6 (8/6), 7-6 (7/5) victory over Milos Raonic on Tuesday.

Despite having a spiritual guru in his camp these days, Djokovic has been anything but serene lately and once again the defending champion cut an angst-ridden figure for long periods of a tense clash in the prestigious season-ending event at London's O2 Arena.

Djokovic had reacted furiously after being grilled by the media about a potentially dangerous incident when he slammed a ball into the stands during his win against Dominic Thiem on Sunday.

This time, the world number two grumbled over camera flashes from the crowd and held a finger to his lips to urge them to be quiet, while moaning when Raonic tried some gamesmanship by taking his time tying up his shoelaces at a key moment in the first set tie-break.

The 29-year-old, who has now won 20 of his last 21 matches at the Tour Finals, would also regain the number one ranking from Andy Murray if he leaves south London with the silverware.

"I managed to hang in there mentally and stay strong and believed that the opportunities would come and that I could take them," Djokovic said.

“It could have gone either way this match. He's such a strong player and has got firepower."