MELBOURNE, Jan 24 - Rafa Nadal, aiming for a fourth consecutive grand slam at Melbourne Park, faces his toughest challenge so far on Monday when he takes on Croatia's Marin Cilic in the fourth round of the Australian Open.
Only four active players boast a winning record against Nadal and the 15th-ranked Cilic is one of them.
More worrying, however, for the Spanish world number one than defeat in his only career meeting with Cilic in 2009 is his health after complaining of "sweating more than normal" in a laboured third round victory against teenager Bernard Tomic.
Nadal also said he felt tired during that match and he will need to be playing at full tilt against the dangerous 22-year-old who lit up last year's tournament with a run to the semi-finals.
Andy Murray, the man who defeated Cilic in that semi-final before losing to Roger Federer in the final, takes on 11th-seeded Austrian Jurgen Melzer at Rod Laver Arena.
Murray has steamrolled his first three opponents but will face a much stiffer test against the late-blooming Melzer, who reached his first grand slam semi-final at the French Open last year as a 29-year-old.
Fourth seed Robin Soderling has also blasted his way to the fourth round with ease and should have too many weapons for 46th-ranked Ukrainian Alexandr Dolgopolov.
In the women's draw, U.S. Open champion Kim Clijsters will face Russia's Ekaterina Makarova in the evening session, while world number two Vera Zvonareva plays Czech Iveta Benesova at Hisense Arena.
Petra Kvitova, who dumped Australia's Sam Stosur out in the third round to sink the hopes of a nation, plays Italy's Flavia Pennetta.