Cincinnati - South Korea's Chung Hyeon rallied past a struggling Jack Sock for a 2-6, 6-1, 6-2 victory on Tuesday and a place in the second round of the Cincinnati Masters.
Chung handed the slumping American an eighth straight defeat, with Sock's last win coming in the opening round at Rome three months ago.
The Korean, who pulled out of Toronto last week with injury, prevailed after Sock took a long medical timeout in the second set for an apparent back problem.
Sock discussed the situation with officials before opting to play on, but Chung charged through the remainder of the match to set up a showdown with fourth seed Juan Martin del Potro.
Unseeded Milos Raonic advanced past Serb Dusan Lajovic 6-3, 6-3 at the combined ATP and WTA event, the last major tune-up prior to the US Open on August 27.
In the women's draw, Australian 16th seed Ashleigh Barty defeated Czech teenager Marketa Vondrousova 6-3, 7-5, while 14th-ranked Elise Mertens topped Magdalena Rybarikova 6-4, 6-2.
Maria Sakkari followed the winning example of Greek compatriot Stefanos Tsitsipas, Toronto finalist against Rafael Nadal, by beating Indian Wells champion Naomi Osaka 6-3, 7-6 (10/8).