Roger Federer, of Switzerland, celebrates after defeating Philipp Kohlschreiber, of Germany, 6-4, 6-2, 7-5, in a fourth-round match at the U.S. Open tennis tournament in New York, Monday, Sept. 4, 2017.
Thiem also aided del Potro by playing his worst tennis when he was closest to victory in the fourth set, which he led 5-2.
His back had been bothering Federer before the U.S. Open and restricted his practice time, something he blamed for problems while getting pushed to five sets in each of the first two rounds last week.
Dolgopolov, who has been interviewed by anti-corruption investigators about a match with unusual betting patterns last month at another tournament, was hesitant to give Nadal too much credit.
That winner faces Estonia's Kaia Kanepi, who beat Daria Kasatkina 6-4, 6-4 to become the first qualifier in 36 years to reach the U.S. Open women's quarterfinals.
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