The 25-year-old seven-time Grand Slam winner made the shocking announcement last Wednesday. She said that after being beaten at the recent German Open by Russia's Dinara Safina: "At the end of the match in Berlin, (retirement) all of a sudden was there."
She added that she had no motivation left to play at the highest level, "And once I lost that, I lost many things."
Sharapova, who topped the prestigious ranking list two and a half years ago, was ranked second last week and moved up when the new list was published on Monday.
Last Saturday Sharapova withdrew from the Italian Open semifinal match against Jelena Jankovic with a strained calf.
The top-ten WTA ranking list includes three more Russians: Svetlana Kuznetsova (4), Anna Chakvetadze (6), Yelena Dementyeva (7).