Doha, Qatar ? Venus Williams defeated her sister, 5-7, 6-1, 6-0, on Thursday to reach the semifinals of the WTA Tour’s season-ending Sony Ericsson Championships, a match Serena Williams called her worst of the year.

Venus Williams won on her fifth match point and is 3-0 in round-robin play of a tournament that features the top eight players. Serena Williams dropped to 1-1, but can still advance to the semifinals if she defeats Elena Dementieva today.

The sisters are 9-9 in career matchups. Serena defeated Venus in the U.S. Open quarterfinals en route to the title this year, and Venus won their title match at Wimbledon.