Juan Martin Del Potro
World ranking: 7
2012 Australian Open: Quarter-finals
Of all the challengers outside of the top four at the Australian Open this year, Argentine Juan Martin Del Potro can lay a claim to enjoying the most Grand Slam success, having won the US Open back in 2009. The powerful server is more than capable of holding his own against the best; he defeated both Nadal and Federer on the way to US Open glory.