Williams, currently ranked 26 in the world, will play the Grand Slam semi-final against Latvian Anastasija Sevastova, 18th in the WTA rankings who beat US defender Sloane Stephens 6-2, 6-3 earlier on Tuesday, reports EFE news.
The former tennis number one and 17th seed in this tournament scored a win in her 100th match on the court of the main stadium in Flushing Meadows, a number no one can match right now.
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Williams used her powerful serve to beat Pliskova, eighth seed, but met resistance and made more unforced errors than the Czech, who was watched from the stands by the Spanish former professional and Wimbledon winner Conchita Martinez, her trainer.
After the match, Williams, 36, said she was playing "freely" because a year ago she was giving birth to her first child and now had "nothing to prove".