US qualifier Steve Johnson, ranked 142nd in the world, shocked top seed Tommy Haas on Wednesday to reach the quarter-finals of the ATP Delray Beach Open.
Johnson, a two-time US collegiate champion, came through a third-set tie-break to overcome world number 12 Haas 6-4 2-6 7-6 (7/2) - only his second-ever career victory over a top-20 player.
Haas - who converted only three of his 12 break opportunities - double-faulted to fall 3-1 down in the breaker and never got back on terms as he slipped to a surprise defeat.
"In the third set, I kept my composure, didn't give him much and played a great 'breaker," said a delighted Johnson. "Tommy is an unbelievable player and this is a win I won't forget."
Johnson will now face Spanish sixth seed Feliciano Lopez rallied to eliminate Frenchman Adrian Mannarino 4-6 6-4 6-2.
Kevin Anderson, bidding for his second title at the event in three seasons advanced to the last eight when Ivo Karlovic retired with a stomach virus.
Hewitt pulls out
The South African fourth seed held serve in the opening game and was poised to break when the big-serving Croatian called it quits.
Karlovic was coming off a run to last Sunday's ATP final at Memphis, where he lost to Japanese top seed Kei Nishikori.
Next up for Anderson is Australian Marinko Matosevic who progressed when his opponent, compatriot Lleyton Hewitt, was also forced to withdraw during their match.
Matosevic had taken a tight first set on a tie-break, but the contest progressed no further as Hewitt - Delray champion back in 1999 - pulled out with a shoulder injury.
Croatian seventh seed Marin Cilic reached the second-round after beating Germany's Benjamin Becker 6-1 6-3.
Cilic, a champion at Zagreb and runner-up at Rotterdam last week, will next meet home favourite Ryan Harrison.