USA's Madison Keys hits a return to USA's Jessica Pegula (not in photograph) during the US Open tennis tournament women's singles round of 16 match at the USTA Billie Jean King National Tennis Center in New York City, on Sept 4, 2023. (PHOTO / AFP)

NEW YORK - Madison Keys stunned third seed Jessica Pegula 6-1 6-3 in an all-American clash on Arthur Ashe Stadium on Monday, reaching the US Open quarter-final for the first time since 2018.

Keys crashed out of the first round at Cincinnati last month but appeared right at home on the New York hard courts, where she was runner-up in 2017, as she overwhelmed her opponent with 21 winners.

Pegula beat the 17th seed Keys in their only previous meeting last year and was considered as one of the US brightest hopes after winning the Montreal title last month.

However, on Monday she lacked her usual firepower and finesse.

Jessica Pegula, of the United States, serves to Madison Keys (not in photograph), of the United States, during the fourth round of the US Open tennis championships, on Sept 4, 2023, in New York. (PHOTO / AP)

"I've had so many amazing moments in New York," said Keys, who is playing in her 12th US Open main draw.

"Being able to at any moment come back from any difficult positions I've been in matches has been amazing."

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She next plays Wimbledon champion Marketa Vondrousova, who overcame a rocky start to beat American Peyton Stearns 6-7(3) 6-3 6-2.

Keys broke her opponent's serve with a backhand winner in the second game of the first set and broke Pegula again on the fourth try in game six.

Madison Keys of the United States is congratulated after defeating Jessica Pegula of the United States during their Womens' Singles Fourth Round match on Day Eight of the 2023 US Open at the USTA Billie Jean King National Tennis Center on Sept 04, 2023 in the Flushing neighborhood of the Queens borough of New York City. (PHOTO / AFP)

Pegula was clearly off her game and threw her racquet down in frustration as she helped Keys to the break with a double fault and a backhand error in the fifth game of the second set.

Pegula broke back immediately in the sixth game but Keys kept her cool, retaking the lead as she broke her opponent's serve in the seventh game.

Keys, who counts Pegula as a close friend, pumped her fist in a subdued celebration as the top-ranked American dropped her serve again when she sent a forehand shot into the net on match point.

READ MORE: WTA: Madison Keys upsets Coco Gauff in Eastbourne

"It's always tough having to play a friend... when we get on the court it's all business," she said in courtside interview.