Djokovic is Nadals biggest barrier to success - Wilander

The Spaniard has struggled with injuries in recent years and endured a disappointing 2015, failing to progress beyond the quarter-final stage at any grand slam.

However, the 14-time grand slam winner displayed impressive form with wins over Stan Wawrinka and Andy Murray at the ATP World Tour Finals before elimination to eventual winner Djokovic in the semi-finals.

Wilander believes Nadal will be back to challenging at the big tournaments next year, but stresses world number one Djokovic will prove difficult to topple.

Speaking to Omnisport at the Champions Tennis tournament at the Royal Albert Hall, he said: "2015 was a pretty decent year for any normal human being. For Rafa, he ran into Novak Djokovic at the French [Open], if he doesn't run into him in the quarter-finals, maybe in the final he has a chance to win again.

"It turned out to be not the greatest year, but I think he's back, in terms of the respect in the locker room is back. He's back in terms of he's not going to lose to too many players that are ranked lower than him.

"Is he going to beat Novak Djokovic in a tennis match again? That's the big question, because really that's the person who he has to figure out.

"It's all about can he win a slam and can he beat Novak Djokovic on the way to do that? I don't think we know that until he's actually done it."

Despite overcoming Nadal in the quarter-finals of the French Open, Djokovic suffered defeat to Stan Wawrinka in the final.

And Wilander thinks missing out on the calendar Grand Slam means the Serbian will be motivated to get off to a winning start to 2016.

"I think for Novak winning the Australian Open is going to be very important because once you start the next year and you're not winning, everyone thinks you're going to win because you won it five times before," Wilander added.

"If he wins that I think he has another serious shot at winning all four [grand slams]. Obviously if he doesn't win it then you wonder what's going to happen to his motivation, because his motivation is extremely high every day of the year and it's tough to keep that up.