Prakash Amritraj upset fourth seed and world number 109 Vincent Spadea in straight sets to advance to the singles quarter-finals in the US $50,000 ATP Challenger in Tallahassee, United States.
The Indian Davis Cupper, who achieved a career-best ranking of 185 last week, stunned his American opponent 6-4, 6-1 in the second round clash.
He will next take on another American in seventh-seed John Isner in the last-eight stage match. Isner defeated Sweden's Bjorn Rehnquist 6-1, 7-6(4) in his second round match.
In doubles, both Prakash and Somdev Devvarman lost their first round matches.
Prakash and Briton Joshua Goodall went down 3-6, 1-6 to the top-seeded American pair of Eric Butorac and Scott Lipsky.
Somdev and Treat Conard Huey of the Philippines fought valiantly but lost 4-6, 7-6(3) 7-10 to American wild card holders Phillip Simmonds and Jesse Witten.
Harsh Mankad is the only Indian surviving in the doubles event. He has already made it to the quarter-finals with his Austrian partner Martin Fischer.