An out-of-sorts Bernard Tomic was dumped out of the Istanbul Cup second round in little over an hour by Diego Schwartzman on Thursday.
Top seed Tomic, handed a bye for round one, is ranked some 65 places above the Argentine in the ATP world rankings.
However, the Australian slumped to a 6-2 6-2 loss in one hour and four minutes, with Schwartzman breaking Tomic's serve on six occasions in a clinical display to claim the biggest scalp of his career.
Schwartzman's reward is a quarter-final tie against Damir Dzumhur, who overcame Illya Marchenko 6-3 5-7 6-3.
Third seed Ivo Karlovic slugged it out with Dusan Lajovic in a 7-6 (13-11) 7-6 (9-7) to record the 300th ATP Tour victory of his career.
Big-serving Karlovic had failed to win in his last eight matches, with the 36-year-old plagued by a knee injury and, although he failed to muster a single break point during regular play, his composure in the breakers proved crucial.
Rounding off the action, fifth seed Marcel Granollers beat Hyeon Chung 6-3 6-4.