Tennis: Venus beats Serena in Qatar

DOHA, Qatar (AP) — Venus Williams defeated her sister 5-7, 6-1, 6-0 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.

Serena started to unravel early in the second set.

"I just couldn't keep a ball in play," she said. "Everything was off."

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 on Friday.

Dementieva defeated Dinara Safina, 6-2, 6-4, in Thursday's last match, eliminating Safina from the championships.

The Williams' 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.