London - Rafael Nadal said he would have to redouble his efforts to return to form after the worst year of his career if he is to get the better of world number one Novak Djokovic next year.
Nadal's world ranking sunk to number 10 in 2015 when his long reign as French Open champion ended, before embarrassingly early exits from Wimbledon and the US Open.
But the Spaniard, who has 15 Grand Slam titles to his name, is enjoying a resurgence in form and said his defeat to Djokovic in the World Tour Finals semis on Saturday would not dishearten him as he prepares for 2016.
"My next step is just keep practising the way that I am doing. I know I need to keep improving things if I want to have chances against Novak next year," the 29-year-old said.
"That's what I am going to try. Just keep working the way that I am doing. I think I am doing well.
"Very few moments in my career I get frustrated. When one player is better than you, the only thing you can do is try to find (a) solution."
The world number five beat Andy Murray, David Ferrer and Stan Wawrinka in the group stages at London's O2 Arena in the season-ending World Tour Finals, disputed between the world's top eight players.
But a red-hot Djokovic proved a class apart, winning their semi-final clash 6-3, 6-3.
"I'm happy about the tournament. Happy about finishing the season with that positive energy. Now I have a week off, and that's it," said Nadal.
The former world number one is eyeing some relentless training and playing in Asia's International Premier Tennis League showcase next month, in order to hit the ground running at the Australian Open in January.
"I practise thinking about what I have to do to be better, then we'll see if I am good enough," he said.
"My motivation has always been just personal, that I want to improve myself, I want to be better. That's what I'm going to try to keep doing. We'll see. 2016 is going to be a new year. Hopefully a better one for me.
"My goal is try to keep being in that top league, that is the top five, top four of the world. That's my goal, and we'll see if I am able to keep doing this."
It has taken the left-hander several months to rediscover a semblance of his best form.
But the Spaniard has no intention of letting his strong showing in southeast London convince him to take it easy before the 2016 campaign begins.
And he predicted Djokovic, who took over from Nadal as world number one in July 2014, may be due a dip in form.
"Novak for the moment is almost unbeatable. The only thing you can do is congratulate him," he said.
"But... you never know what's going on in the future. I have been in very good situations a couple of times, then I get some injuries, things change.
"What Novak is doing is just amazing. But it's obvious that it's true if somebody can do it, it's him, but it's not easy to stay at that high level four or five years in a row."