NEW DELHI: Spaniard Rafa Nadal remains fiercely loyal to his uncle and coach Toni despite saying he does not know when, or even if, he will win another grand slam title.

Former world number one Nadal failed to add to his 14 major titles this year as he missed out on any of the sport's four grand slams for the first time since 2004.

The 29-year-old endured a difficult year and slumped to 10th in the rankings in June, although he showed glimpses of his old power in the latter weeks of the season, beating world number four Stanislas Wawrinka and number two Andy Murray at the ATP World Tour Finals and finishing the year ranked five.

"I don't know when that's going to happen and I don't know if that's going to happen," Nadal, currently in New Delhi to represent the Indian Aces in the International Premier Tennis League, told reporters on Thursday.

The likes of seven-times grand slam champion John McEnroe have suggested Nadal could use the help of a new coach, but the Mallorcan seems unlikely to go down that route.

"Without Toni, nothing would have been possible," he said. "I feel so lucky that I have somebody from my family that is my coach for ever. That's a big advantage in my opinion.

Toni's response was a light-hearted one.

"It's not my decision, his decision. No, maybe he needs the best coach, but I am not too expensive," he said.

Great rivals Roger Federer and Novak Djokovic have both talked up Nadal's chances of adding to his grand slam collection. Nadal says he will just keep working hard to "create opportunities".

"2015 has been not the best season possible. The last three months, I think it was much better," Nadal, who reached finals in Beijing and Basel and the semis at the ATP's year-ender in London where he lost to Djokovic, said.

"I'm happy the way that I'm playing now. I need to keep going. I want to keep going the same way that I finished the 2015 season. I think that I improved a lot so much in the last tournament.

"That's the way I'm going to keep working hard to try and continue with that level of the game."