Tim Henman suffered his third defeat in four matches this year when Guillermo Canas fought back to win their first-round clash at the Miami Masters.
Henman will next play in the Davis Cup against the Netherlands
Canas, 29, took his record against 32-year-old Henman to six wins in seven matches with a 3-6 6-2 6-1 victory.
Henman took an early 4-1 lead on his way to the first set and broke early in the second before Canas hit back.
The Argentine levelled the match with two breaks and dominated the decider as Henman failed to win any service games.
"Obviously my game plan was to get to the net at every opportunity, and especially in the windy conditions," Henman told the ATP website.
"I think I wanted him to hit as many passing shots as I could, and for a little bit, I played some fantastic tennis.
"But it doesn't take a great deal for that to turn around and I think a combination of my first serve percentage dropping a little bit, and then him starting to pick up his passing shots and return a serve....
"You know, the balance begins to shift."
The Argentine has enjoyed a fine run since returning from a 15-month drugs ban in September, winning the Brazil Open last month and ending Roger Federer's unbeaten run two weeks ago.
"I played very well the first tournaments and then I get good confidence in my game," said Canas.
"Week by week, I'm getting better in my tennis, in my mentality, in my performance."
He goes on to face Juan Carlos Ferrero in round two, while Henman must turn his attention to Great Britain's Davis Cup tie against the Netherlands on 6 April.