Thirtysomethings Roger Federer and Novak Djokovic share the Australian Open record with six wins, but either could make history with a magnificent seventh when the first Grand Slam of the year begins in Melbourne on Monday.
The pair face a stern challenge from youthful force Alexander Zverev, looking for a first major as the torch-bearer for the next generation.
But there are question marks over the fitness of Rafael Nadal and Andy Murray, meaning it remains to be seen if all members of the "Big Four" will again prove a force to be reckoned with.
But since winning a fourth Wimbledon in July the Serb rose inexorably back to number one by losing only three further matches -- one of which was to Zverev at the ATP Finals.
Djokovic won his third US Open in September to put him on 14 Grand Slams -- three behind Nadal and six behind Federer.
Ageless Swiss master Federer, now 37, will remain a chief threat to the Serb but the other two members of the Big Four look to be struggling after an injury-plagued 2018.
- NextGen 'already there' -
"I think if we are healthy and playing well, the four guys still have probably the best chance to always win Slams," said Djokovic.
The Serb warned however that "the next generation is already there" and picked young guns Zverev of Germany, Borna Coric of Croatia, Karen Khachanov of Russia and Greece's Stefano Tsitsipas as key threats to the Big Four, who have won a staggering 49 of the last 56 Grand Slams stretching back to 2004.
"It's just a matter of time when we will see some of them competing in the last stages of Grand Slams," said Djokovic.
Zverev, 21, starts the Australian Open full of confidence after an impressive warm-up in Perth's mixed teams Hopman Cup, despite his terrible record at Grand Slams.
The German has never got beyond the third round in Melbourne.
Federer seems to thrive at the start of the year and begins his campaign on the back of winning the Hopman Cup. He is hunting a third successive Australian Open after last year becoming the oldest world number one in the 45-year history of the ATP rankings.
- Competitive at 37 -
"(Pete) Sampras once upon a time said, 'If you win a Slam, it's a good season'," said Federer, now ranked three, who skipped Roland Garros and had disappointing exits at Wimbledon and the US Open.
"I played super well in Australia again. So obviously I can't wait to go back there. I'm very proud that at 37 I'm still so competitive."
Murray's hip surgery meant he played just six events in 2018, and Nadal's truncated 2018 campaign was bookended by injuries that forced to him to retire in both the Australian and US Opens.
And Cilic, who pushed Federer all the way in the final last year, heads into the tournament after pulling out of a warm-up event in India last week with an injured knee.
Home fans will look to Alex de Minaur and Nick Kyrgios, but the latter is searching for form after a troubled 2018 and tumbled out of the Brisbane warm-up, where he was defending champion, in the second round.
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