Stosur, fresh from victory over four-time champion Justine Henin on Monday, produced another fighting performance to defeat World No. 1 Serena Williams at the Rolland Garros quarterfinals 6-2, 6-7 (2), 8-6.
The Australian's epic victories have seen her price sucked in to $2.15 on TAB Sportsbet from $75 when she was trailing Henin by a set in the round of 16.
"We already know this is a monumental effort by Sam, but it is put into greater context when you consider she's been written off by punters around the world in her last two matches," The Daily Telegraph quoted Mathew Thompson, Betfair spokesman, as saying.
"However, there were some loyal Stosur supporters that took the tournament winners odds of $75 against Henin and the $20 against Serena and they've been rewarded handsomely," he added.
Stosur is only the third Australian to beat a World No.1 after Jelena Dockic beat Kim Clijsters in Zurich in 2003 and Nicole Bradtke beat Steffi Graf in a 1993 Fed Cup tie.
She will now play Jelena Jankovic in the last four, while Francesca Schiavone faces Elena Dementieva in the other semis.
Meanwhile, Serena follows her sister, Venus, Henin and Caroline Wozniacki, on the way home or to the UK to get ready for Wimbledon.
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