Halep beat the eighth-seeded Keys with powerful groundstrokes to set up a semifinal match against Petra Kvitova, who beat Johanna Konta 6-3, 6-4.
''It was a great match for me,'' Halep said. ''I knew she would be very tough because she's strong and serves really well. But I was moving well and enjoyed my time on the court.''
With the win, Halep improved to 4-1 against Keys and qualified for her third straight appearance at the WTA Finals in Singapore.
It was Halep's third victory against Keys this year following wins in the third round of Wimbledon and in the final of the Rogers Cup in Montreal.
Svetlana Kuznetsova came from match point down to defeat third-seeded Agnieszka Radwanska 1-6, 7-6 (9), 6-4.
''I was getting better as the match was going on,'' Kuznetsova said. ''Aga played well in the first set, but I wasn't moving so well.
''In the second set I wasn't playing my best game. But I was trying to stay in there, trying to fight, trying to be smart and turn things around.''