US Open 2016

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.

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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Johnson played his last seven holes knowing he had to review a possible rules infringement after the round.

He shot 68 to finish five under but was then penalised a shot for making his ball move on the fifth green, despite being initially absolved of wrongdoing.

McIlroy called organisers "amateur" and Spieth said the decision was "a joke".

Johnson's score was amended to four under, but he still finished three shots ahead of Irishman Shane Lowry and American duo Jim Furyk and Scott Piercy at Oakmont.