Pliskova, 24, won 6-2 7-6 (7-5) to set up a first Grand Slam final against Angelique Kerber or Carolina Wozniacki.
Williams' defeat guarantees that Kerber will overtake the 34-year-old at the top of the rankings after the US Open.
It will end a 186-week run as number one for Williams, which ties Steffi Graf's WTA record.
Williams had also been hoping to pass Graf's Open-era record of 23 major singles titles in New York, but for the second year running she fell at the semi-final stage.
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