Rafael Nadal described Novak Djokovic as "almost unbeatable" after the world number one produced a masterclass to reach the final of the ATP World Tour Finals at the Spaniard's expense.
Nadal has appeared to be getting back to his best at the O2 Arena this week, but felt the force of an inspired semi-final display from Djokovic on Saturday.
Djokovic has been a cut above his rivals this year, winning 10 titles - including three grand slams - and needing just one more victory on Sunday against either Roger Federer or Stan Wawrinka to triumph in the season-ending tournament in London for the fourth year in a row.
Fourteen-time grand-slam champion Nadal gave credit where it was due to the top seed following his latest scintillating performance.
When asked what has made Djokovic so difficult to beat, Nadal said: "Everything. He is hitting amazing. The return is always amazing. This year he's serving great, I think.
"And then he is able to play with no mistakes and change direction so easily, playing so, so long. He's doing everything good.
"He was better than me and he deserved to do what he did during the whole season. He played just fantastic. When somebody's doing this, the only thing I can do is congratulate him and not wish him the best of luck for the next year!
"Novak for the moment is almost unbeatable."
Nadal was encouraged by his performances this week - having won all three round-robin matches - and is determined to raise his game next year after a frustrating 2015.
He said: "I think I improved a lot over the last couple of months.
"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. That's what I am going to try. Just keep working the way that I am doing.
"I think I am doing well. I am happy about the tournament. I'm happy to finish the season with that positive energy. Now I have a week off, and that's it."
He added: "I just accept that the opponent is better than me today and keep working hard to try to reduce that difference, because he cannot improve much. I think I could improve a lot.
"I was able to compete with everybody. That's a big improvement for me.
"But that's not enough for me. I want to keep going and I want to feel better next year. I am going to work hard to make that happen."