Several of Thursday afternoon's matches were suspended in progress, with Novak Djokovic leading fifth seed GrigorDimitrov 2-6 6-3 2-1 and Marin Cilic tied with Karen Khachanov at 7-6(5) 3-6 1-1.
Wild-card entrant Stan Wawrinka was up 2-1 in the first set against Marton Fucsovics.
In third round action, big-serving Milos Raonic beat fellow Canadian Denis Shapovalov 7-6(6) 6-4 while Spanish 13th seed Pablo Carreno Busta topped Dutchman Robin Haase 6-4 6-2.
"I think I got myself into more points on his serve than he did on mine,” said Raonic.
"It was the thing I struggled with most last time we played, so I definitely made sure to focus on that and sort of try to change that a little bit."
Del Potro and Chung were delayed from taking the court because of rain and then had to endure another weather-related suspension just six minutes into their match.
Del Potro was not rattled by the delays, however, and did well to attack Chung's forehand throughout the 81-minute encounter that determined the final entry into the last 16.
"I am happy to beat these young players," del Potro said. “He has a big future, and he is a nice player. I am glad to go through."
This marks Del Potro's fourth consecutive last-16 appearance in Cincinnati, where the Argentine appears to be in fine form right in time for the Aug. 27-Sept. 9 U.S. Open.
(Reporting by Frank Pingue in Toronto, editing by Pritha Sarkar/Peter Rutherford)
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