Novak beat Andy Murray 3-6, 6-1, 6-2, 6-4 to complete the career Grand Slam, and became just the eighth man to accomplish the feat.
Novak completed dream season capturing a career-high 11th title of the year and fifth ATP World Tour Finals trophy (b.Federer 6-3, 6-4)
The world no.1 claimed a record fourth BNP Paribas Masters title on Sunday with a 6-2, 6-4 victory over no.2 seed Andy Murray.
Novak captured his eighth ATP World Tour title of the season and 56th of his career over no.3 seed Rafael Nadal 6-2, 6-2, in the final of the China Open on Sunday.
This is Novak's fifth runner-up finish in Cincinnati, following his Final appearances in 2008-09 and 2011-12.
Novak successfully defended his Championships title, defeating seven-time former winner Roger Federer in four sets 7-6 (1), 6-7 (10), 6-4, 6-3.