The top-ranked Serb has not lost at the season-ending tournament at the O2 Arena since 2011 and has won three straight titles since then.
The world number one needed just 65 minutes to see off the eighth seed Nishikori, and ended the afternoon with a trophy in his hands as he was confirmed as the 2015 world number one.
Before Sunday’s game, the ATP held a minute’s silence for the victims of the Paris attacks.
Elsewhere, Maria Sharapova came back from a set down to win her reverse singles match against Petra Kvitova to give Russia a 2-1 lead in the Fed Cup final in Prague.
In Sunday’s clash, the world number four, playing in her first Fed Cup final, defeated Kvitova 3-6, 6-4, 6-2 in the battle of the teams’ top singles players.
Kvitova, ranked number six in the world, broke the Russian in the first game and again in the ninth to take the set 6-3.
In the second set, Sharapova broke Kvitova to lead 3-2 before the Czech broke back in the next game to level at 3-3.
Sharapova broke again to lead 5-4 with a fine backhand pass and she closed out the set 6-4.
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