American John Isner was dumped out of the Argentina Open after suffering a 7-6 (12-10) 4-6 7-6 (8-6) loss to Dusan Lajovic on Wednesday.
Isner, the world number 18, hit 15 aces during the two hour, twenty minute contest but it was the Serbian who came out on top in Buenos Aires.
Lajovic was able to save five of six break points in a superb display from the world number 84.
The 25-year-old will now face fifth seed Dominic Thiem in the quarter-finals after the Austrian ousted Gastao Elias 3-6 7-6 (9-7) 6-3.
Elias took out the first set after breaking twice, but Thiem hit back in the second despite his Portuguese opponent fending off five break points.
However, Elias could not keep up in the deciding set as Thiem closed out the match.
Joining Thiem and Lajovic in the next round was Jo-Wilfried Tsonga after the third seed eased past local Leonardo Mayer.
The match lasted just under an hour as Tsonga cruised to a 6-1 6-2 victory.
Tsonga will face 2011 champion Nicolas Almagro in the quarter-finals after the Spaniard defeated Federico Delbonis 6-3 4-6 6-4.
Almagro hit five aces to one against his Argentinian opponent to book his spot in the last eight.