World number 56 Arthur Gea stunned Ben Shelton in straight sets in the third round of the Shanghai Masters on Sunday as the US Open finalist became the biggest casualty so far.
The 21-year-old Frenchman beat Shelton 7-6 (7/5), 7-6 (7/3) for the biggest win of his career.
"I got outplayed today," world No 4 Shelton told reporters.
"My body wasn't ready," he said, playing in Shanghai as his first tournament since the US Open, apart from the Davis Cup last month.
He skipped the Japan Open that ended last week due to fitness concerns.
"That's just the way this tour is sometimes," he said, adding that jet lag had also taken a toll.
Gea will play either Francisco Cerundolo or Jakub Mensik next.
"Amazing, to be honest," he said after his first win over a top-10 player.
He said his game was "all about focus, concentration" and staying "ready for every point".
Gea added that he has been "more confident on court" following a breakout year that saw him clinch his first professional title in July at a 250-level event in Los Cabos.
He stormed to a 5-3 lead in the first set before the 24-year-old American Shelton came back fighting, levelling to 5-5 with a clutch backhand volley at the net.
The two players were neck and neck in the tie-break until Gea sneaked ahead to win the first set point.
Third seed Shelton forced his way into another tie-break in the second set as the match went past two hours, but faltered as a charging Gea clinched victory at his first chance.