Haas will take on Ricky Barnes in Saturday's semifinals. Barnes defeated 18-year-old Spencer Levin 4 & 3 in the quarterfinals.
Hunter Mahan, the 1999 U.S. Junior Amateur champion, bested Henry Liaw 3-and-2 Friday to reach his first semifinals at the U.S. Amateur. Next up for Mahan is Dustin Bray, who advanced after knocking off Ryan Moore 4-and-3.
Haas caught fire immediately Friday with six birdies in his first eight holes, and in the process grabbed a 3-up lead. At the par-3 ninth, Haas landed his ball 20 feet from the hole and ran home the birdie try to take a commanding 4-up lead to the back nine.
Haas won the 10th to go 5-up but Klauk fought back. Klauk captured the 11th with a short birdie putt and capitalized on a mental error by Haas at the following par-5.
Haas found a greenside bunker in two at the 12th and blasted out to the fringe, 17 feet from the hole. Klauk sank a 25-foot birdie putt, then won the 12th when Haas chose to pick up his ball instead of attempting his birdie chip to halve the hole.
'It was pretty stupid,' said Haas, who attends Wake Forest University. 'I was dumbfounded after I did it.'
The mistake didn't hurt Haas as he went on to win the next two holes to end the match on the 14th.
'I am a little disappointed that I lost,' said Klauk, who won this year's Western Amateur. 'He flat-out beat me today.'
Barnes pounced on Levin early with a birdie-birdie-eagle run starting at the fourth. Although Levin hit a fine approach to 12 feet at the sixth, Barnes bettered the shot by knocking his second down the ridge on the green and into the cup for an eagle.
'By the sound of the roar, I knew it was in,' said Barnes, who closed out the quarterfinal match up on the 15th hole.
Mahan trailed 2001 U.S. Junior Amateur champion Liaw after five holes but turned the tables and grabbed the lead for good by winning the sixth and seventh.
Bray cooled the hottest player left in the championship. Moore won his two matches on Thursday 7-and-5 and 5-and-4, respectively, but did not have the same kind of success with Bray.
The match went back and forth until Moore bogeyed the ninth to fall 2-down. At the 11th, Moore chipped in to save par and seemingly earn a halve, but Bray holed a birdie putt to win the hole and go 2-up.
'I came out today and things just didn't go right. That's how it goes sometimes,' said Moore.