Marcos Giron cruised past top-seeded Frances Tiafoe 6-1, 6-4 on Friday to set up a semi-final clash with fourth-seeded Adrian Mannarino of France at the ATP Dallas Open.
Giron, ranked 67th in the world, notched his first win over a top-20 player since he beat Canadian Felix Auger-Aliassime to reach the Tokyo semi-finals in October.
World number 14 Tiafoe couldn't muster a break point against his fellow American, who won their first career encounter at the Paris Masters in 2021 but had lost their next three meetings.
Mannarino, who received a walkover in the second round, was leading 6-2, 3-1 when Aussie opponent James Duckworth retired with an injury.
Saturday's other semi-final will be an all-American affair between second-seeded Tommy Paul and third-seeded Ben Shelton.
Paul edged closer to a second ATP title with a 7-5, 6-3 victory over Germany's Dominik Koepfer while Shelton defeated Jordan Thompson 6-7 (2/7), 6-4, 6-4.
Paul and Koepfer traded five breaks of serve in the opening set, Paul breaking at love in the 11th game and serving out the set.
The 15th-ranked Paul needed five match points to put it away, squandering a 40-0 lead before closing out the final game.
Shelton dropped the first set despite not facing a break point, but he came up with gritty service holds to finish off the second and third sets.
He fended off four break points in the final game to seal it on his second match point.