For Novak Djokovic, the main question heading into 2016 is how he can possibly top what he achieved over the past 12 months. The Serbian enjoyed one of the most dominating years in the history of tennis, eclipsing even his own performance in 2011, when he didn’t lose a single match until the semifinals of the French Open in June.

The bare facts this time around make for astonishing reading. In 2015, Djokovic won 11 titles, one more than four years ago, finishing with a win-loss record of 82-6, putting his winning percentage of 82.6 sixth on the all-time list on the ATP Tour. He suffered just one defeat against a player ranked outside the top 10, and recorded 31 victories over those inside it.

And he delivered on the big occasion, too. Remarkably he won six of the nine Masters 1000 events, as well as adding the season-ending ATP World Tour Finals title. Most significantly, he held aloft three of the four Grand Slam trophies, with his wins at the Australian Open, Wimbledon and the U.S. Open taking his Grand Slam haul to 10 and putting him seventh on the all-time list.

What was most impressive about his season was his unwavering physical and mental performance. In the finals of each of the three Grand Slams he won, Djokovic faced a competitor -- Andy Murray in Australia and Roger Federer at Wimbledon and the U.S. Open -- who pushed him all the way in tying up the first two sets. But from that point on, Djokovic’s refusal to break and to keep pummeling away from the back of the court simply ground his opponent down.

There was, Djokovic will need no reminding, one notable exception. His only defeat in the four Majors in 2015 came in the final of the French Open, the one Grand Slam tournament he has yet to win. After beating the man who has dominated Roland Garros for the past decade, Rafael Nadal, in the quarterfinals, Djokovic looked set to finally get his hands on the Coupe des Mosquetaires, only to be beaten by a superb performance from Stan Wawrinka. Djokovic’s emotional response afterward spoke to the crushing disappointment of failing to become just the eighth man to complete the Career Grand Slam.

And making up for the loss is likely to be his top priority in 2016. But the whole year represents an opportunity for Djokovic to truly put himself in the discussion for the greatest player of all time. Given his dominance of the past 12 months, even winning a clean sweep of all four Grand Slams cannot be ruled out, and that would put him level with Rafael Nadal and Pete Sampras on 14 titles. Then only Roger Federer, currently on 17, would be ahead of him.

At the age of 28, there is little to suggest that Djokovic will be slowing down anytime soon. So what can stop him in 2016? This year he finished the year with double the ranking points of third-placed Federer with more than 7,600 more than second-placed Murray.

Federer again could be a threat, even into his 35th year. But while the Swiss triumphed twice in finals, Djokovic has shown repeatedly that he has the edge over five sets. The same is true of Murray. The Scot looks set to have a good year after focusing heavily on the Davis Cup in 2015, and he is able to call upon the memory of beating Djokovic in the final of two Grand Slams. However, generally over five sets it has been Djokovic who is able to maintain his level, while Murray dips in and out. Certainly Djokovic will not be counting out Wawrinka, who has shown in a one-off match he has the game to hit anyone, including the world No. 1, off the court.

The big unknown is the man who Djokovic has faced more than anyone in his career -- Nadal. After his worst year in more than a decade in 2015, the Spaniard showed signs of regaining his belief at the end of the year. If he can stay physically fit, Nadal is perhaps the only man who can go toe-to-toe with Djokovic over five grueling sets.

Another potential hurdle is the schedule. With the Rio Olympics coming up in August, between Wimbledon and the U.S. Open, it is set to be a hectic summer for the top players, increasing the chance of some surprise results. In truth, though, the odds look very good that Djokovic will remain a firm No. 1 and add to his burgeoning Grand Slam collection.