Rankings lose relevance for injured players. Vika has only played 13 tournaments in the last year, including Wimbledon and IW. Sharapova probably 11 or 12, you can't get ranking points if you're not able to compete in tournaments.
I always think it's strange when people are surprised at players with long term injuries dropping in the rankings. Petkovic dropping out of the top 100, zvonareva 900 or whatever. Rankings are just a method for seedings and only useful as a comparison of players if they have similar numbers of counting tournaments.
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