Rafael Nadal is not certain he will return to his very best form and full fitness in 2023 – but he has no doubt he will give it his very best shot.

The veteran Spaniard went down 6-3 6-4 to Felix Auger-Aliassime on Tuesday at the ATP Finals, his second defeat in the round-robin group stage and a fourth consecutive loss overall.

This season began in stunning style for Nadal as he won the Australian Open and followed that with a 14th French Open title, taking his tally of grand slams to 22 - a record haul for a male singles player.

However, Nadal has a serious ongoing foot problem, and the 36-year-old had to withdraw from Wimbledon prior to a semi-final against Nick Kyrgios after suffering a torn abdomen.

Injuries have prevented him playing a full season, and he was clearly not at his best against Auger-Aliassime, who has Nadal's uncle, Toni Nadal, on his coaching team.

Asked whether he will take an optimistic or doubtful approach into next season, Nadal said: "You can be both ways. You can be optimistic and have doubts. Why not?

"For me the doubts, as I said plenty of times, are very good in this world. People who don't have doubts, it is probably because they are too arrogant, from my point of view.

"If you are not optimistic or positive, it's impossible that things are going to go the proper way. That's my point of view and that's my approach."

He will hope to make a strong start to the new campaign in Australia, and will put in the work to give himself every chance of showing up in shape to win again.

"I just need to recover all these positive feelings and all this confidence and all this strong mentality that I need to be at the level that I want to be," Nadal said.

"And I don't know if I'm going to reach that level again. But what I don't have any doubt about is that I'm going to die for it."

He will round off his round-robin campaign against Casper Ruud in Turin on Thursday, facing the young Norwegian he crushed in the French Open final.

Once he returns to his Mallorca home, Nadal can look at what lies ahead.

"What can happen in Australia? I don't know. It's a month and a half away," Nadal said.

"What can I do to give myself a positive chance to have a good season next year? As always, be humble enough to accept that I have a challenge in front, that the last six months have been very difficult for me.

"I need to work more and I need to recover things that I lost because I was not able to practice the proper way, I was not able to compete the proper way.

"What I have to do now is come back, work hard, stay positive every single day, accept the challenge, accept that I am going to need to suffer a little bit more."