Top seed Dominic Thiem was knocked out of the opening round of the Antalya Open by a qualifier on Tuesday.
Ramkumar Ramanathan, ranked 222nd in the world, was able to dispose of Thiem 6-3, 6-2 to reach his first ATP 250 quarter-final of the year.
The Indian star took advantage of of an out of sorts Thiem early on, running out to 3-0 lead on his way to taking the opening set in just 29 minutes.
It was more of the same in the second set, with Ramanathan earning a double break enroute to the biggest win of his career.
World No.222 beats world No.8! ?
Ramkumar Ramanathan shocks Dominic Thiem in Antalya… pic.twitter.com/RDxPOOouPg
— Tennis TV (@TennisTV) June 27, 2017
"I just gave it my best and was serving well out there," the 22-year-old told atpworldtour.com.
"Two or three points can often make the difference on grass. It's a good start for my career, but it's just a start. Hopefully I can keep it going."
Elsewhere, third seed Fernando Verdasco eased past Steve Darci 6-3, 6-4 and former world number eight Marcos Baghdatis battled past Carlos Berlocq 6-4, 3-6, 7-5.