The 'Big Four' is set to become the 'Big Three' and it appears Roger Federer and Novak Djokovic will be hard to stop at the Australian Open.

Federer and Djokovic will be aiming for a seventh title in Melbourne – a total that would give either the most by any man of all time.

Just a year ago, questions were being asked about Djokovic, while Federer entered the year's first grand slam in 2017 on the back of an injury lay-off.

This time, there's uncertainty over Rafael Nadal, while Andy Murray will bid farewell to Melbourne in 2018.

Nadal, 32, has talked up his form, but his recent record on hard courts is concerning and the 17-time grand slam champion pulled out of the Brisbane International due to a thigh strain. He played three hard-court tournaments in 2018 and he completed just one of those – winning the Rogers Cup in August.

Nadal's last competitive match was at the US Open, where he retired in the semi-finals due to a knee injury, before undergoing ankle surgery in November. While he played the Fast4 event in Sydney, Nadal's preparation leaves plenty of doubts over whether he can win a second Australian Open title and first since 2009.

Of the 'Big Four', there were only bigger question marks over Murray, who announced on Friday he would retire no later than this year's Wimbledon.

The Brit played just 12 singles matches last year – winning seven – after having surgery on his hip, before ending his season in September. Now the world number 230, Murray enters the Australian Open with a protected ranking but unseeded. He beat wildcard James Duckworth at the Brisbane International before losing to Daniil Medvedev, but his hip is causing him too much pain to continue his career beyond 2018.

A five-time Australian Open finalist, the 31-year-old deserves to be remembered for his runs in Melbourne regardless of what happens this time around.

The 'Big Four' have won 53 of the past 60 majors, and 13 of the previous 15 titles in Melbourne. But the responsibility of them improving that record appears set to fall largely to Djokovic and Federer, who have won eight of the previous nine Australian Opens.

While unable to add to his 20th major after winning in Melbourne last year, Federer enjoyed another fine season, albeit with some stumbles. The Swiss great lost to Kei Nishikori and eventual champion Alexander Zverev at the ATP Finals, while he suffered two defeats to Borna Coric and was also beaten by Australians Thanasi Kokkinakis and John Millman in 2018.

But for the third year in a row, Federer prepared at the Hopman Cup, again showing good signs, particularly in wins over Zverev and the up-and-coming Stefanos Tsitsipas. Federer carries a 14-match Australian Open winning streak into the tournament, with his last defeat against Djokovic in the 2016 semi-finals.

Djokovic may be the man to beat after recapturing his best form in the second half of last year.

The Serbian 14-time grand slam champion won Wimbledon and the US Open, a trip to the French Alps revitalising the superstar. He has been unable to win his past three events, including in Doha, where he lost to Roberto Bautista Agut in the semi-finals.

Since winning back-to-back titles in Melbourne in 2015 and 2016, Djokovic has been surprised by Denis Istomin and Hyeon Chung in the past two years.

If the 'Big Four' are to begin 2019 in dominant fashion once more, it seems it will be up to Djokovic or Federer to ensure they do so.