Rafael Nadal had one word to describe Novak Djokovic's performance in their semi-final clash at the Barclays ATP World Tour Finals: "unbeatable".

Djokovic notched a comprehensive 6-3, 6-3 victory over his Spanish rival on Saturday, turning in an unrelenting baseline game and nearly untouchable serving performance. Nadal discussed the Serb's dominant run following the match.

"Novak for the moment is almost unbeatable," the Manacor native said. "The only thing you can do is congratulate him. But things are like this today. What Novak is doing is just amazing. It is 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 years in a row or five years in a row.

"He's hitting amazing. The return is always amazing. This year he is serving great. And then he is able to play with no mistakes and changing directions so easy, playing so, so long. He's doing everything good."

Nadal concludes his 2015 campaign with a 61-20 mark, including three titles from six finals in Hamburg, Stuttgart and Buenos Aires. He secured his fifth semi-final berth in seven trips to the season finale following a perfect 3-0 record in Group Ilie Nastase.

Does Djokovic agree with Nadal's assessment? The Serb, who notched his eighth win in his last nine encounters against Nadal, was hesistant to embrace the "unbeatable" status.

"I don't feel unbeatable, but it does definitely feel great when I'm playing the way I played today, when I feel like I'm dictating the pace, most of the rallies, I feel like I'm on top of my opponent," Djokovic said. "There are days when you don't feel great, where you have to find a way to win.

"It's hard to say or point out something that separates me from other players. Every player has his own pros and cons. I feel like over the years I managed to improve my transition from the defense to offense. My serve has gotten better, not in terms of speed, but in terms of precision and accuracy. That was obviously one of the parts of my game that worked well today. I got more than a few free points on the first serves.

"I know I can rely well on my defence, but I also wouldn't like necessarily to stay in the back of the court for most of the match. I know that the matches against the top rivals at this level are won when you are aggressive and when you're taking the ball early. The first shot, serve, being aggressive and constructive in the right points, that's what I focus on."

Djokovic will square off against a Swiss opponent in Sunday's final - Roger Federer or Stan Wawrinka. He is bidding to complete a historic four-peat at the season finale, having captured the previous three titles.