Let's be honest. If Serena Williams is at her best in Flushing Meadows, no one can touch her. But the best still have mental lapses and physical slumps, and Serena has had to grind out a few tough wins during her 2011 hard court streak, sometimes against lesser names on the tour.
While the biggest threat to Serena might be simple exhaustion from winning too much, even if that's a non-issue then there are still players who can give her a true challenge. Let's look at the six who have the best chance of pushing her to the limit and maybe even claiming victory.