Top players are vulenerable in the first round, agreed?
Lower ranked players can only have the chance to outplay the top players in the first round.
Some top players are likely to be out in the first round, perhaps they're not warmed up fully.
Sharapova - 2007 Australian Open 1st round 6-3, 4-6, 9-7
Venus Williams - 2007 Wimbledon 1st round 2-6, 6-3, 7-5
Serena Williams - 2007 French Open 1st round 5-7, 6-1, 6-1
Justine even lost to Eleni Daniilidou in 2005 Wimbledon first round 7-6(8), 2-6, 7-5