LONDON -- When it was suggested he has not been his normal self, physically or emotionally, for the last six months, Novak Djokovic bristled.

"What makes you say that?" he replied, indignantly.

The question came straight off the bat at his pre-tournament news conference Friday afternoon at the O2 Arena. The four-time defending champion, who kicks off his World Tour Finals Sunday against Dominic Thiem, perhaps felt he should be afforded more respect.

Djokovic had been getting used to arriving in -- and leaving -- London as world No. 1; he has finished the year on top of the rankings for four of the last five seasons.

He had led Andy Murray by more than 8,000 points after beating the Scot in the French Open final in June, completing not just a career Grand Slam but holding all four major titles at the same time with his 12th overall.

His form dramatically tailed off, though. There were shock early exits at Wimbledon and the Rio Olympics, before a US Open final defeat to Stan Wawrinka in New York -- where he had looked out of sorts the whole fortnight, and greatly benefited from three walkovers.

After missing Beijing, where he had won 29 straight matches, a semifinal defeat in Shanghai and a quarterfinal exit at the Paris Masters ensured Murray overtook him in the rankings by reaching the final at Bercy last weekend.

The mitigating circumstances for his slump? A mixture of injuries, "personal problems" as he admitted at Flushing Meadows, and a natural drop off after Roland Garros.

"I'm my normal self every day," Djokovic continued Friday. "First of all, I was talking about having certain ups and downs that everybody has. Not [being] myself emotionally or physically the last few months is too rough of a statement.

"I thought I had a good couple of months, maybe not up to the standards of 12 to 15 months before that. In sport you can't always expect to win.

Let me play you a song and make you smile :) 🎶🎵London here I come! A video posted by Novak Djokovic (@djokernole) on Nov 10, 2016 at 12:01am PST

"The high standard of results and success I've had the last couple of years probably has taken its toll. I didn't get to recover I guess as fast after the French open, to be ready to compete on the highest level after that.

"I had to dig deep and take some time to reflect on everything because it was an incredible achievement. It took a little bit of time to get back on track.

"I played well at the US Open I thought, made the finals, winning Toronto. The injury took me off from the Olympic Games but all in all a very good year that I'm proud of. I'm here in London to try to crown this year with the best possible result."

Djokovic's latest run at the summit finally came to an end Monday after 122 weeks -- only Jimmy Connors, Ivan Lendl and Roger Federer have enjoyed longer consecutive streaks at No. 1.

It took an incredible run from Murray to reel him in, with victories at Queen's Club, Wimbledon, the China Open, Shanghai, Vienna and Paris taking the Scot's title count to a personal-best eight for the season.

"I have only words of praise for what he [Murray] has achieved in the last year or so," added Djokovic. "Starting with the clay court season, he was very consistent.

"He had the best one of his life, playing three finals of three big events and winning one in Rome. He won every event he played on grass including a Grand Slam, and then the Olympics -- he's had an incredible run.

"Definitely well deserved No. 1 at this point. He's been the best player of the last six months without a doubt. Whether or not he can sustain it is not a question for me.

"The qualities he possesses as a player, his determination to the sport, there's a good chance that he can still play in this level for some time."

Novak Djokovic spoke to the media at the O2 Arena Friday. JUSTIN TALLIS/AFP/Getty Images

On paper, the Serb should not have too much trouble making the semifinals here -- he is a combined 23-0 against the other three players in his group, Milos Raonic, Gael Monfils and Thiem.

Djokovic can reclaim top spot if he goes one better than Murray in London, while there are several other permutations that could see either man crowned year-end No. 1, making for what could be a thrilling finale to the season.

But he insists he doesn't need any extra motivation.

"Even after 10-plus years at a professional level, I still feel I have a lot of gas in my tank," said Djokovic. "I feel like I still have a lot more years ahead of me. I feel great joy when I compete, when I play.

"The power of passion towards the sport is the No. 1 thing that drives me to play, to enjoy to play. This is the game. if you don't enjoy the game, your time on court, everything else becomes two or three, five times more difficult than it is or should be.

"You do have days when you're feeling maybe less determined to play. Some days you feel it more, but as long as the days that are when you're waking up and feel more positive are there making the majority of those days, then things are going well. That's the case with me.

"Practice days make probably 80 percent of our season. That's where you really need to get that inspiration and drive going on a daily basis, so you can maintain that level of determination and competition at the highest levels, at tournaments like this. Fortunately for me at this stage I don't have that problem,

"I have plenty of motivation -- it starts from within. My mindset is very positive towards this sport, this life that I have. I have great support of my family, my wife. It's important. Otherwise the things would not be as smooth. As long as it's like that, I'll be around."