(AFP) - Rafael Nadal cast doubt on whether he could win another Grand Slam after his long reign as French Open champion ended in 2015 and his rankings slid during the worst year of his career.

The 29-year-old Spaniard said he was working hard to return to form and was happy with the way the year ended after a resurgence at the World Tour Finals in London in November.

But asked whether he could clinch another major title to add to the 14 Grand Slams he has already accumulated, Nadal was unconvinced when speaking to reporters in New Delhi.

"I don't know when that will happen and I don't know if that's going to happen," he said.

"The only thing I know is I'm working very hard to try to get the opportunities."

Nadal was speaking after hitting balls with youngsters during a coaching session in the Indian capital where he plans to open an academy for talented players.

Nadal sunk to world No. 10 this year after losing at the French Open quarter-final stage, before suffering early exits from Wimbledon and the US Open.

But Nadal, who has been plagued with injuries and last year had surgery to remove his appendix, rallied in the latter part of this year to finish at world No. 5.

"2015 has been not the best season," he said.

"(But) In the last three months, I finished much better and I'm happy the way I'm playing now. I need to keep going the same way that I finished the 2015 season."