The American's 186-week reign as world number one is over after the 6-2 7-6 (7-5) loss to Czech Pliskova at Arthur Ashe Stadium on Thursday.
Williams, whose bid for a 23rd grand slam also came to an end, said a knee injury had been impacting her from the opening rounds of the tournament.
"I have been having some serious left knee problems," she said.
"I wasn't tired. Fatigue had absolutely nothing to do with it. If I was tired I should definitely get into a new career."
Williams added: "I wasn't able to move the way I wanted to move.