Second seed Tommy Haas suffered a first round defeat in the Delray Beach International on Monday, losing to Argentine Diego Hartfield in three sets.
Hartfield, who had never won a match in an ATP hardcourt event, triumphed over the German 6-2, 3-6, 6-2.
Haas made a series of unforced errors, particularly with his backhand and attempted drop shots.
He was playing his first competitive game since undergoing shoulder surgery on November 10.
"This was my first test, I have progressively started stepping it up over the past 10 days but the shoulder left me at the end of the second set," he told reporters.
"There was not really any pain but the power wasn't there, the muscle wasn't ready.
"It was a big test, I thought I could get close to winning but I didn't expect great things to be honest."
American Robby Ginepri took the first step in his bid to climb back up the rankings with a 6-3, 6-0 win over Frenchman Mathieu Montcourt.
Ginepri, a US Open semi-finalist in 2005, has slumped from 15th to 169th in the past three years.
Against Montcourt he dropped only six points on serve and did not face a break point on his way to clinching his first win in 2008.
"It's been a while since I've felt like that on court," Ginepri told reporters. "It felt like the old Robby."
