Kwon Soon-woo upstaged second seed Milos Raonic 7-6 (7-4) 6-7 (4-7) 6-4 to reach the New York Open quarter-finals, while Borna Coric exited the Argentina Open.
Raonic advanced to the Australian Open quarters before falling to eventual champion Novak Djokovic last month, however, the Canadian was stunned in New York on Wednesday.
Kwon weathered 33 aces from Raonic as the South Korean prevailed after two hours, 25 minutes at the ATP 250 tournament midweek.
"I'm really happy to win a tough match against Raonic," Kwon during his on-court interview. "He played well. It was a great match."
Next up for Kwon is either eighth seed Kyle Edmund or Dominik Koepfer.
While Raonic bowed out, fellow seeds Ugo Humbert and Miomir Kecmanovic progressed from the last 16.
French fourth seed Humbert eased past Marcos Giron 6-1 6-0, sixth-seeded Serb Kecmanovic defeated Paolo Lorenzi 6-3 6-3, while Andreas Seppi saw off Steve Johnson 7-6 (7-4) 6-3.
At the Argentina Open, fourth seed Coric was sent packing by Thiago Monteiro – who won 6-4 7-6 (8-6) in Buenos Aires.
Despite letting a match point slip at 6-5 in the second-set tie-break, Monteiro moved through to his first ATP Tour-level quarter-final since 2018.
Pedro Sousa awaits after the lucky loser accounted for Jozef Kovalik 7-6 (7-3) 7-6 (7-5) on the clay courts.
Dusan Lajovic – the third seed – outlasted qualifier Pedro Martinez 7-6 (8-6) 7-6 (8-6) to set up a quarter-final with eighth seed Casper Ruud, who dismantled Roberto Carballes Baena 6-1 6-0.