MUMBAI (REUTERS) - Novak Djokovic's domination of 2016 is not as inevitable as it might appear, former British No. 1 Tim Henman believes, even if the other members of the "Big Four" will have to raise their game and hope the Serbian falters.

Djokovic won 11 titles including three of the four Grand Slams last year and has a 12-0 record this season, having thrashed Rafael Nadal, Roger Federer and Andy Murray to win the title in Doha and a record-equalling sixth Australian Open crown.

So impressive was the 28-year-old at Melbourne Park that many pundits believe a first calendar year Grand Slam in the men's game since Rod Laver in 1969 is on the cards.

Henman, while conceding there is now a gap between Djokovic and the other top players, does not think it has become a gulf.

"They can definitely challenge. Murray is at the highest ranking in his career, he's number two in the world. He's playing great tennis," the 41-year-old Henman said in an interview. "But obviously Djokovic is playing a little bit better at the moment.

"Roger was in the final of US Open last year and he was in the final of the Wimbledon last year. Djokovic stopped him in his quest to win more Grand Slams. They are still very, very close."

Henman thinks Federer, who won the last of his 17 Grand Slam titles in 2012, and Murray look most likely to prevent a Djokovic Grand Slam sweep this year as Stan Wawrinka did at the French Open last year.