There are certain players in the game that are always a threat to the top players, regardless of their ranking status. In the 1990s, Peter Korda, Karol Kucera and Richard Krajicek were not consistently ranked high, but their mercurial natures did not prevent them from upsetting Sampras and Agassi on the biggest stages in the world.
Korda was ranked 16th when he knocked out Pete Sampras at the 1997 US Open. Kucera was outside the Top 20 when he beat Sampras and Agassi when they were defending their Australian Open and Roland Garros crowns, respectively.
Krajicek wasn’t in the Top 10 when he thumped Sampras in straight sets at Wimbledon and finished his career with a combined 9-8 head-to-head record against Agassi and Sampras.
On the other hand, there have always been relatively high ranked players who are quite benign on the big stages. Today, there’s a few players lurking around the top that are hardworking athletes, but when they come up against the elite crop, they don’t seem to have much of a chance. David Ferrer, for instance, never looked like a legit threat at the recent ATP Tour World Finals.
There’s no question that there are a few other players currently ranked low, that the top players rather not face at the Australian Open than David Ferrer (7), Mikhail Youzhny (9) or Jurgen Melzer (11).
While Ferrer and company are established players who can hang with the best, they don’t have the game or the big match temperament to consistently challenge a Djokovic or Federer at, say, the Rod Laver Arena.
On the other hand, there is a player like Nalbandian, who shuffles in and out of the Top 10 like Mike Tyson in and out of prison. But Nalbandian thrives on the big stage and is only marginally less menacing than a boxer, which has less to do with his surly attitude as his meticulous point construction.
I compiled a list of other big time players, currently ranked outside the Top 10, who have shown the temperament and game to cause at least two big upsets in their career. Players like Querrey and Monfils may well go on to be bigger threats next year, but this list only includes those who have shown their lack of fear on the big stage—even if for short periods.