It seems like Serena Williams will need either a wild card or use her protected ranking to be able to play at the US Open 2011. Serena’s ranking is currently 175 in the world and this would mean that she might get into qualify with this ranking.
If a player is out with an injury for more than six month they can receive a protected ranking. Thus, when they return to tour they can use this ranking to get into tournaments they were previously playing. When Serena received her protected ranking she was the world number 1.