Novak Djokovic proves a challenge too far for resurgent Rafael Nadal

— -- LONDON -- Rafael Nadal has enjoyed an encouraging resurgence at the ATP World Tour Finals this week but Novak Djokovic was simply too good here, as he has been for nearly all of his most glorious season.

The world No.1 is through to a record 15th consecutive final on Sunday, a run dating back to his Australian Open win in January.

He will face either Roger Federer or Stan Wawrinka, after a clinical 6-3, 6-3 semifinal win at the O2 Arena. It was close to perfection from Djokovic.

Nadal said after the match that the Serb is "almost unbeatable" when in this mood. "I don't feel unbeatable," said Djokovic, "but it does definitely feel great when I'm playing the way I played today."

He added: "It's the best season of my life. Regardless of the outcome of tomorrow's match, the season will still be the best that I had. There is always room for improvement, so that's something that honestly excites me and motivates me to keep on going."

Djokovic's shock straight-sets defeat to Federer in the group stages, which snapped his 23-match win streak, looks to have been a mere blip.

He has bounced back with two victories, against Tomas Berdych in his final round-robin match and Nadal in this semifinal, to take his record for the season to 81-6 -- that is a win rate of over 93 percent.

Djokovic will look to crown one of the finest seasons since the Open era began with what would be a record fourth consecutive World Tour Finals title and fifth overall, having also won in Shanghai in 2008. It is now eight defeats in nine for Nadal against the Serb.

Their 46 meetings are the most between any two players during the Open era, with Djokovic tying their head-to-head for the first time at 23-23 after 10 years.

"I was a few levels under him at the beginning of my career when I started playing professional tennis," said Djokovic. "Nadal was alongside Federer dominating the tour. I just couldn't really do much against him. But because we played so many times I had a chance to really shorten the gap, and now even the score."

Nadal denied he was feeling the effects of his long match against David Ferrer on Friday, but Djokovic certainly looked fresher after an extra day's rest. Nadal's first serve deserted him straight away as the world No.1 pounced by reeling off four winners -- three forehands and one backhand -- to break to love for a 2-0 lead.

The Serb, stepping into the ball to stifle the aggression Nadal had shown in his impressive wins over Stan Wawrinka and Andy Murray, closed out the first set and then broke for 3-2 in the second.

Again, he didn't look back, breaking once more at 5-3 to make it four wins out of four against the Spaniard in 2015.

"He was better than me and he deserved to do what he did during the whole season. He played just fantastic. The only thing I can do is congratulate him and just wish not the best of luck for the next year," Nadal said with a smile.

After a season of self-doubt that saw him slip to No.10 in the world in June -- for the first time since 2005 -- Nadal can be proud of the way he has finished it.

He has been reinvigorated and his confidence renewed in London. There may have been no Masters or grand slam titles to celebrate this year, but Nadal has put together his best ever sequence following a U.S. Open (17-5).

By sweeping his group, Nadal suggested he could once again be a force to be reckoned with in 2016.

"I need to keep improving things if I want to have chances against Novak next year," Nadal added. "That's what I am going to try. I think I am doing well. I'm happy about finishing the season with that positive energy.

"Things are like this today. 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. Not everything is that easy.

"What Novak is doing is just amazing. 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."

Djokovic proved to be one hurdle too far for Nadal, but that has been the case for most players who have come up against him in 2015.

The world No.1 has lost just six times, and four of those in finals, becoming the first player to win 80 matches in a season since Nadal in 2008. The Serb leads the Tour with 10 titles and won his eighth, ninth and 10th grand slams this season.