Top seed David Nalbandian was knocked out of the first round of the Acapulco Open late on Tuesday, losing 1-6, 6-3, 6-4 to unseeded Austrian Daniel Koellerer.
Nalbandian swept through the first set in 31 minutes but fell to pieces after that, producing a string of unforced errors as 116th ranked Koellerer charged into the second round.
"It's the biggest win of my career, I can't believe it's happened," Koellerer told ESPN television.
"I'm going to celebrate but I'm also going to talk to my coach to see where I went wrong in the first set."
World number 12 Nalbandian also suffered a shock defeat to fellow-Argentine Juan Monaco in the semi-finals of the Buenos Aires Open on Saturday.