Sisters Serena and Venus Williams will face each other for the third time in a Wimbledon final.
Defending champion Venus beat Russia's Elena Dementieva in straight sets 6-1 7-6 (7-3) to reach her seventh final.
And Serena knocked out China's Zheng Jie, also in straight sets, with a score of 6-2 7-6 (7-5).
The American sisters will play together in Friday's doubles semi-final, before meeting in the Women's singles final on Saturday.
Venus said despite the competition, the sisters still rely on each other for support: "We definitely rely on each other's advice even more...I think we both now know our roles in the relationship and we support each other."
On Friday the men's semi-finals are due to take place. Roger Federer will take on Marat Safin while Rafael Nadal will meet Rainer Schuettler.