Williams beat the 20-year-old Keys 6-3 6-3 in 68 minutes to book a 27th professional career showdown with her older sister, who advanced with a 6-2 6-1, 50-minute fourth-round victory over qualifier Anett Kontaveit, 19, of Estonia.
Three-time defending champion Serena holds a 15-11 advantage in the sisterly head-to-head series against Venus, splitting their four previous clashes at the U.S. Open.
"She's playing great," 33-year-old Serena said about seven-times grand slam winner Venus, who is two years her elder. "I have to really be ready for that.
"At least one of us, a Williams, will be in the semis."
Top-seeded world number one Serena has won six of the last seven matches against Venus, who beat her younger sister last year in the semi-finals in Montreal.
Venus was quick to identify her biggest challenge in facing Serena in Tuesday's quarter-final.
"Well, obviously, that she's so good. What else can you do except try to win the point and hope she doesn't hit an ace,"
Venus said.
Serena said it was always tough facing her sister.
"I'm playing, for me, the best player in the tournament, and that's never easy," she said. "She's beaten me so many times ... more than anybody.
"She's a player that knows how to win, knows how to beat me, and knows my weaknesses better than anyone.
"So it's not an easy match at all. Hopefully things will go right."
Venus, whose last slam triumph came at the 2008 Wimbledon, said she thought she was playing well enough to win her third U.S. Open title. "Of course. I'd love to," she said. "But it's easier said than done."
Serena brought her 'A' game against Keys, dropping just six points on serve in the first set.
Serena cracked six aces and never faced a break point in beating the power-hitting Keys, who put up a strong fight in their only previous meeting, in this year's Australian Open semi-finals.
Keys lost 7-6(5) 6-2 in Melbourne, saving seven match points in the second set before succumbing.
This time, Serena played her strongest match of the tournament so far and Keys contributed to her loss.
Keys double faulted on the last two points of the eighth game for the lone break of the first set, and double faulted again on match point.
Serena, winner of 21 grand slam singles titles including the 2015 Australian Open, French Open and Wimbledon championships, currently holds all four grand slam titles, a "Serena Slam" she has now twice accomplished.
Winning this time at Flushing Meadows for her seventh U.S. Open crown would give her the rare calendar-year Grand Slam that would match Maureen Connolly (1953), Margaret Court (1970) and Steffi Graf (1988).
(Editing by Andrew Both)
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