Sergiy Stakhovsky sent seventh seed Joao Sousa crashing out in the first round of the Kremlin Cup after recording a 6-4 6-3 victory in Moscow.
The world No 98, who famously defeated Roger Federer at Wimbledon this year, needed just an hour and 17 minutes to defeat an opponent ranked 48 places above him.
The Ukrainian went 4-1 down in the opening set but dominated the contest from there on top set up a second-round clash with Edouard Roger-Vasselin, who beat Jiri Vesely 6-3 6-7 (3-7) 6-3.
"I started a bit nervous and experienced certain problems in the beginning," Stakhovsky said.
"But later in the first set I managed to bring my concentration back and dominated the play in the rest of the match."
Elsewhere in the first round, Sixth seed Horacio Zaballos fought back from a set down to beat Ricardas Bernakis 1-6 6-3 6-4, while Russian teenager Karen Khachanov defeated Albert Ramos 6-2 3-6 6-4.