US qualifier Steve Johnson, ranked 142nd in the world, stunned top seed Tommy Haas on Wednesday to reach the quarter-finals of the ATP Delray Beach Open.
Johnson, a two-time US collegiate champion dominated a third-set tiebreaker to beat world number 12 Haas 6-4, 2-6, 7-6 (7/2) for his second career victory over a top-20 player.
Haas converted only three of his 12 break opportunities. Johnson, who saved all seven break points he faced in the opening set, pounced on Haas's miscues in the tiebreaker, when the German veteran double-faulted to fall 3-1 down, then sailed a backhand wide to give Johnson a commanding lead.
"In the third set, I kept my composure, didn't give him much and played a great 'breaker," said Johnson, who next faces sixth-seeded Spaniard Feliciano Lopez. "Tommy is an unbelievable player and this is a win I won't forget."
Lopez advanced with a 4-6, 6-4, 6-2 victory over France's Adrian Mannarino.
Another quarter-final was set as South African fourth seed Kevin Anderson and unseeded Australian Marinko Matosevic advanced against ailing opponents.
Matosevic was leading fellow Aussie and eighth seed Lleyton Hewitt 7-6 (7/2) when Hewitt retired with a right shoulder injury.
Hewitt had received treatment on the shoulder during the first set.
Anderson, bidding for his second Delray Beach Open crown in three seasons, advanced when Karlovic retired with a stomach virus.
Anderson, who beat Matosevic in the 2012 Delray final, held serve in the opening game and was poised to break when the big-serving European 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.