Venus has not played a top-level singles match since Wimbledon
Venus Williams has withdrawn from next week's WTA event in Montreal because of a left knee injury that has frustrated her US Open preparations.
The world number four also withdrew from this week's event in Cincinnati.
She will join her sister Serena in missing the two top warm-up events for the year's final Grand Slam tournament that begins on 30 August in New York.
"The pain that I am experiencing in my knee has unfortunately not gone away," Williams said.
Venus Williams would have played in Montreal for the first time.
Instead, the seven-time Grand Slam champion has not played a top-level singles match since she was ousted from the Wimbledon quarter-finals by 82nd-ranked Tsvetana Pironkova of Bulgaria on 29 June.
The only remaining US Open warm-up event possible for either Williams sister would be at New Haven in the week before the US Open.
Serena Williams had to have surgery on her right foot after she cut it on broken glass and missed events in Istanbul, Cincinnati and now Montreal.