The upsets continue at the Auckland Classic tennis event, with second-seeded American John Isner losing to compatriot Steve Johnson in three sets.
Isner had said he was playing with house money after coming within a whisker of defeat on Wednesday and No.7 seed Johnson collected on Thursday, winning 6-3 5-7 7-6 (7-3).
While 208cm Isner's powerful first serve remained a weapon, it was when he was forced into second serves that Johnson pounced.
The 27-year-old Johnson consistently pushed inside the baseline to return and during rallies to put the pressure on world no.19-ranked Isner under a sunny Auckland sky.
Johnson forced Isner to save nine of 10 break points as he broke in the first set to win it and, after losing the second, threatened to break again early in the third.
But, in the only quarter-final to feature a clash between two seeds, Johnson eventually took his chance to claim the match in the tiebreaker when his forehand nicked the net and pulled up too short for Isner.
The No.2 seed joined other big names, who on Wednesday tumbled out of the tournament.
It started with defending champion and top seed Roberto Bautista Agut pulling out with a stomach bug, while sixth seed Feliciano Lopez also withdrew and No.3 seed David Ferrer crashed out.
US fourth seed Jack Sock won through to the last four after producing an incredible 25 break-point opportunities against Frenchman Jeremy Chardy.
World No.71 Chardy, however, managed to save 21 of the break points before Sock went on to win 5-7 6-4 6-3.
"Obviously, I had a lot of chances early and wasn't able to convert," 24-year-old Sock told the crowd after the match.
"It was pretty upsetting, but I was able to battle through it, find a way and get it done."
Johnson will take on Sock in the semi-finals in an all-American showdown.
Cypriot Marcos Baghdatis beat former champion Jiri Vesely in straight sets to also reach the last four.
The eighth-seeded Baghdatis mixed his shots well on Thursday and was never challenged on serve as he won 6-2 6-4.
He will face an in-form Portuguese world No.44 Joao Sousa in the semi-finals. The 27-year-old Sousa accounted for 58th-ranked Dutchman Robin Haase 6-3 6-2.
