Rafael Nadal and Roger Federer have history against them as they strive to win the season's opening major at the Australian Open in Melbourne.
World number one Nadal is bidding to become the first man in 42 years to hold all four Grand Slam titles simultaneously, but the 'Rafa Slam' is the subject of some debate.
Australian legend Rod Laver, who twice accomplished a calendar Grand Slam in 1962 and 1969, contends that what Nadal is pursuing cannot be technically termed a Grand Slam.
"People will say, 'He's going for a Grand Slam.' And I say, 'No, he's not doing that,'" Laver said.
"That wasn't the way this whole thing was set up. It starts in January and ends in September; starts with the Australian Open and ends with the US Open."
The Spaniard strung together the last three majors at the French Open, Wimbledon and US Open to head into the new year poised to continue his dominance of men's tennis.
Nadal became only the seventh man in history -- and the youngest at 24 in the Open era -- to claim a career Grand Slam of the four major tournaments when he conquered Novak Djokovic in New York last September.
Nadal is reluctant to dwell on his chances of pulling off a 'Rafa Slam' over the next fortnight, especially as he is up against the all-time Grand Slam leader Federer.
"I think if that's to happen, for sure I'll be more happy that I won the Australian Open than because it is the fourth one in a row," he said.
"Maybe I'm only going to have this opportunity once in my career.
"But not for that reason I am going to have the pressure. The pressure is like every Grand Slam, you want to play well in the important tournaments and for me, to have the fourth (major) or not is something that is not in my mind."
Tennis traditionalist Federer also believes a Grand Slam should be won in a calendar year.
"For me it's the one during the year. Then the other one, if Rafa were to do it, it's amazing in itself as well. It's just a different order," Federer said.
"But it shouldn't take much away from it. It's very close, but it's not the calendar Grand Slam... Obviously, the classic one is the one that Rod Laver did twice. That will always be that way."
Nadal and Federer have dominated the Grand Slams and in the last six years only three men -- Juan Martin del Potro (2009 US Open), Novak Djokovic (2008 Australian Open) and Marat Safin (2005 Australian Open) -- have broken the nexus.
Federer, supplanted by Nadal at the top of the rankings last June, is chasing his 17th Grand Slam title at the Australian Open and has the weight of odds against him.
There have been only nine successful title defences at the Australian Open in the Open Era (post-1968), the least of all four major tournaments.
Federer, who overcame Andy Murray in Melbourne final in 2010, successfully defended his 2006 Australian win the following year.
There have been eight Grand Slam events since the last successful title defence, achieved by Federer at the 2008 US Open.
Federer, now 29, has struck an ominous vein of form, downing Nadal for the ATP World Tour Finals crown in November and winning this month's Qatar Open to go 10 matches unbeaten.
"I won the World Tour Finals, I've been playing really well on the hard courts right now," he said.
"But he's been the one dominating the slams. That makes him the favorite.
"I don't have any problems not being the favorite really."
If there is to be a non Nadal-Federer winner at this year's Australian Open, it is likely to come from third seed Djokovic, number four Robin Soderling or fifth seed Murray.