US sixth seed Sam Querrey was forced to withdraw with a sore back while leading his first-round match at the Delray Beach Open on Tuesday.
Colombia's Alejandro Gonzalez advanced to the second round with the American leading 6-3, 1-2.
"I had a sudden movement at the net and kind of jerked my back a little bit," Querrey said.
"It's an injury that I felt before, but never on a specific motion like this. My lower back just kind of tightened up and I can't really move it in any direction."
Gonzalez will next face American Donald Young, whose hot streak of form -- he reached the semi-finals in Memphis last week -- continued with a 7-5, 7-5 defeat of Croatia's Ivan Dodig.
South African top seed Kevin Anderson and US second seed John Isner were each set to open in night matches against Australians, Anderson facing John-Patrick Smith and Isner meeting Marinko Matosevic.
French fifth seed Adrian Mannarino defeated US qualifier Eric Quigley 6-4, 6-4 while US seventh seed Steve Johnson advanced 6-3, 4-1 when Kazakstan's Mikhail Kukushkin retired due to illness and Serbian eighth seed Viktor Troicki outlasted France's Stephane Robert 6-7 (3/7), 7-5, 6-1.
Two Aussies won early, with Bernard Tomic eliminating Tunisia's Malek Jaziri 6-3, 6-3 and qualifier Thanasi Kokkinakis downing Serbia's Filip Krajinovic 6-2, 6-3.
Two Asian players reached the second round as well, with Japanese qualifier Yoshihito Nishioka beating Dutchman Igor Sijsling 7-5, 6-1 and Taiwan's Lu Yen-Hsun defeating Aussie Sam Groth 6-7 (6/8), 7-6 (7/5), 6-4.