A day after wrapping up the year-end world number one spot, a tetchy Djokovic dropped a set to Nishikori in the semi-finals before winning 6-1 3-6 6-0 at the O2 Arena.
After dropping only nine games in his three group wins, the Serb was on course for another stroll but lost his way.
A double fault while serving at break point down in the second game of the second set was cheered loudly by the capacity crowd, prompting a sarcastic response from the 27-year-old who was clearly riled by the support for Nishikori.
“I mean, look, at the end of the day I cannot blame the crowd,” a weary-looking Djokovic told reporters after taking a long time to arrive for his post-match news conference.
“The crowd has a right to do what they want, to cheer for whoever they want. Some individuals were going over the line throughout the whole match, some provocations that I usually don’t react on, but I did. It was my fault.
“I lost the concentration. I lost the break because of that. I allowed myself to be in the situation to lose the set, maybe even lose the match. I should know better.”
Djokovic, who will face either Roger Federer or Stanislas Wawrinka in Sunday’s final, was asked why he had signed the TV camera lens with a single dot when walking off, rather than his signature as he usually does.
“I felt like that was something I wanted to do today,” he said. Pressed whether it was some kind of message, he added: “No. There was nothing. It’s just a dot.”
Djokovic showed far more emotion after an easy victory over Tomas Berdych on Friday that secured the year-end top ranking for a third time in four seasons than he did in beating Nishikori, admitting he felt emotionally flat.
“Mentally, the truth is that I’m exhausted,” he said. “Honestly, today I found it a little bit difficult to stay concentrated throughout the whole match.
“After the emotional three matches I had, especially yesterday when I achieved the goal to finish as number one in the world, knowing that, I felt a little bit flat emotionally. I needed more time to kind of give myself a boost.
“I was fortunate because in the beginning of the third set, he had break points. If he broke me, the match could have gone either way. I managed to find that little bit of strength.”
(Reporting by Martyn Herman; editing by Ken Ferris)