The early exits of Li Na and Petra Kvitova, French Open 2011 champion and Wimbledon 2011 champion, respectively, in this year's US Open, begs the question once again about the pressures these players undergo when they finally make it!
Svetlana Kuznetsova faced the same problems after she won her 2004 US Open title and took her another 3 years before she won her second Grand Slam in the French Open. After that, she has continued to struggle. Ana Ivanovic is another Grand Slam winner who has been struggling to find her great form after she won the French.
I think this could be a good topic for research among sports psychologists! Of course, there have been consistent winners like the Williams sisters, Kim Clijsters and Justin Henin - at least for the past ten years.
Anyhow, I hope this trend stops since it's really unnerving to see former champions stumble so early in a Grand Slam.