Novak Djokovic was quietly celebrating yesterday a second year as ATP world number one after ending 2012 with a 7-6 (8-6 ), 7-5 win over Roger Federer to earn a second title at the World Tour Finals.

The Serb didn't have it all his way during the season, losing the top ATP ranking to Federer after Wimbledon but taking it back in the closing weeks of a tight season points race.

Djokovic admits that the stress of having a father in the hospital overcoming breathing problems during the last few weeks contributed to the strains of the winning campaign.

"It was a fantastic year, where I've had to face a lot of difficulties off the court as well," said the 25-year-old, who also won the season-ender in 2008.

"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 in the final. This was a title for him."

Djokovic, who dominated in 2011 by winning three of the four Grand Slam titles, said he's had an exciting if challenging 24 months.

"It's been a very long two years, but very successful two years," he said.

"I didn't really know how I will follow up after an incredible 2011, but I believed that I have to use the time where I'm playing the best tennis of my life and I'm winning Grand Slams; finally realize what I need to do to win the major tournaments.

"This was my time, my moment, and I needed to step in and really believe in my ability. So throughout the whole season I've had lots of success and had some disappointing losses at big events, in a couple major finals."

It was not only Djokovic and his team who were celebrating - the player's beloved dog Pierre was also due for a treat. "He's going to get an extra bone tonight, for sure," Djokovic said. "He got a lot of presents actually that we've been enjoying these last couple of days."

Djokovic will barely rest, with his busy schedule now featuring a set of upcoming exhibition paydays before a true holiday.

"In the next six, seven days, I have a couple exhibition events. Then two and a half weeks completely off in a very tropical, very beautiful place with no racket, no nothing, no tennis - just trying to recharge my batteries."

The Serb said that while he did not want to talk too much about his father's health concerns, things were improving.

"It has to stay private in a way because it's a family matter. I don't want people to start talking about my father's health," he said.

"At a certain stage, it was very critical, we didn't know what tomorrow brings. But now he's much better and he's recovering. He's still in intensive care. I'm going to visit him [today] and see how it goes and try to bring a trophy with me and at least make him smile."