Grigor Dimitrov's tough year continued with a first-round loss at the Atlanta Open, while Jack Sock's return ended in defeat on Tuesday.
Dimitrov has struggled throughout 2019 and the former world number three went down to qualifier Kevin King 7-5 6-4 at the ATP 250 event.
The Bulgarian now holds an 11-12 win-loss record this year, a run that has seen him drop to 53rd in the world.
Sock, making his first appearance since January after being sidelined due to a thumb injury, was edged by Miomir Kecmanovic 7-6 (11-9) 7-6 (7-5).
There were three seeds in action, with Frances Tiafoe suffering a surprise 4-6 6-3 7-6 (7-4) loss to Bernard Tomic.
Radu Albot, however, avoided an upset defeat, getting past Tennys Sandgren 6-4 7-6 (7-4), and Ugo Humbert was too good for Cole Gromley 7-6 (7-2) 6-2.
Reilly Opelka sent down 17 aces in a 6-3 7-6 (7-1) victory over Alexander Bublik and Daniel Evans thrashed Jason Jung 6-1 6-1 inside an hour.
Matthew Ebden overcame Kamil Majchrzak 7-6 (7-5) 6-3 and qualifier Kwon Soon-woo edged Prajnesh Gunneswaran 6-7 (4-7) 7-5 6-0.