DOHA, Qatar—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 ...

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