In the unpredictable encounter, Sabalenka was one game away from losing, down 6-0, 5-3, before she turned the match around to take a 3-1 lead in her head-to-head with Keys.
World No.17 Madison Keys cruised into her third US Open semifinal after defeating No.9 and Wimbledon champion Marketa Vondrousova 6-1, 6-4 in the quarterfinals on Thursday.
The American advanced to her ninth major quarterfinal with a 3-6, 7-6(4), 6-2 victory over Andreeva in a match that lasted for two hours and 2 minutes.