Roger Federer and Rafa Nadal are just one step away from a U.S. Open semi-final clash but both face tricky challenges to set up what would be their maiden encounter at Flushing Meadows.

Swiss Federer, chasing a record-extending 20th grand slam title, takes on Argentine Juan Martin Del Potro, the man who beat him in the 2009 final in New York.

While Federer eased into the fourth round, 24th seed Del Potro battled through five sets against Austrian Dominic Thiem and his flat forehand could prove lethal against the former world number one.

Current world number one Nadal faces the impressive Russian 19-year-old Andrey Rublev, a player who has shown no fear in his run to the quarter-finals.

"I have a good shots. I can play really fast," the teenager warned his 31-year-old opponent.

The first match of the day will see women's world number one Karolina Pliskova, who demolished Jennifer Brady 6-1 6-0 in the previous round, face off against another American when she plays 20th seed Coco Vandeweghe.

Madison Keys, one of four Americans in the quarter-finals this year, plays Estonian qualifier Kaia Kanepi, who is back to her best after almost two years off the courts because of an injury and a virus.

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