LONDON – Novak Djokovic said his ailing father’s struggle with serious illness had inspired him to beat Roger Federer in a thrilling climax to the ATP World Tour Finals on Monday.Srdjan Djokovic was absent from his normal courtside position as his son beat Federer 7-6 7-5 at the O2 Arena and cemented his place as the world’s best player.He was not out of mind, however, and Djokovic was quick to pay tribute to his father after his gritty victory over world number two Federer, winner of the title for the past two years."It’s been a very long year, a very long two years, but very successful two years,” five-times grand slam champion Djokovic, who ended a second consecutive season as world number one having won a career best 75 matches, told reporters."I didn’t really know how I would follow up after my incredible 2011, but I believed that I have to use the time where I’m playing the best tennis of my life."It was a fantastic year, where I’ve had to face a lot of difficulties off the court as well. Especially coming into this tournament, having my father fighting his own fight for health gave me extra strength that I wanted to play for him in a way."That’s one of the reasons I really gave it all every match, especially tonight. This was a title for him.”Djokovic’s father has been seriously ill with a respiratory problem and Djokovic has spent hours on the phone over the past eight days monitoring his progress.It makes his achievement of marching to the title undefeated, having won all his round-robin matches, including one against U.S. Open champion Andy Murray, even more impressive.Djokovic battled back from 3-0 down in the first set and 5-3 in the second to claim his first year-end title since 2008 in Shanghai, and with it the $1.76m jackpot for remaining unbeaten.