Novak Djokovic hasn't had any trouble thus far at the Australian Open, but he's about to face some players who have what it takes to oust him.
Of course, Roger Federer and Andy Murray are still alive in Melbourne; as long as that is the case, no one has a better chance to take down the top-ranked player in the world than them.
But there are other threats to the Djoker out there. They are just a cut below that top tier of players, but with their success at the Australian and against Djokovic, they have what it takes to pull off the upset down under.
Djokovic will cruise to the semifinals in Melbourne. Once there, however, he could end up falling to a fellow countryman.
Janko Tipsarević is the ninth-ranked player in the world, second to Djokovic among Serbian players. The 28-year-old Tipsarević has both the skill and experience to take down the reigning Australian Open winner, in no small part because he has done so before.
In seven career meetings, Tipsarević has beaten the world's best twice, including once on a hard court at the 2011 ATP World Tour Finals.
Tipsarević knows Djokovic's game as well as anyone in tennis today. If he can translate that hard court success to Melbourne, he could knock off Djokovic yet again.
If Tipsarević even wants a shot at Djokovic, he'll have to outlast David Ferrer.
Even at age 30, the Spaniard can run with any player in the world. He's as agile as they come and excels in grueling matches that challenge his fitness. Under the hot sun of the Australian summer, that is a crucial skill to have.
Ferrer has also defeated Djokovic twice before, including a hard court win of his own at the 2011 ATP World Tour Finals round robin.
For Ferrer, though, surface is not a huge concern. The fifth-ranked player in the world has a game that is suited for any type of court and any type of opponent. As long as Ferrer stays mistake-free, Djokovic would have a hard time beating him.
If Djokovic makes it to the finals in Melbourne, he may not run into Federer or Murray. Instead, he could end up facing off against an old rival.
Jo-Wilfried Tsonga and Djokovic have met fourteen times on the court, most notably in the 2008 Australian Open finals. It was the first match between the two, in which Djokovic won his first Grand Slam title in straight sets.
In 2010, Tsonga got vengeance against the Djoker in the Australian Open quarterfinals, though Djokovic was dealing with an illness during that match.
With a wealth of experience against Djokovic and a power game perfectly suited for the hard courts of Melbourne, Tsonga can't be considered a second-tier player. He's ranked eighth in the world, but at the Australian Open, he has as good a chance as anyone to win it all.