Félix Auger-Aliassime won the biggest point of the match — then his body gave out. The No. 3 seed and two-time US Open semifinalist was upset by unseeded veteran Karen Khachanov 6-7(5), 6-3, 6-2, 6-2 in the tournament's second round at Louis Armstrong Stadium on Thursday, collapsing physically after taking the first-set tiebreak in front of a stunned Flushing Meadows crowd.

The match ran 3 hours and 16 minutes in roughly 86-degree heat following days of rain. Auger-Aliassime, ranked world No. 4, described the moment things turned in stark terms afterward. "After a set and a half, I just couldn't feel well and was dizzy and seeing spots and couldn't recover," he said, according to the ATP Tour's official recap. "I thought maybe it would pass, but it didn't." He called the sensation "abnormal," distinguishing it from the cramping and fatigue he said he's used to managing on tour: "I cannot beat him or anybody in this draw feeling the way I felt today."

A collapse in the numbers

The statistical picture matched the eye test. Auger-Aliassime committed 47 unforced errors to Khachanov's 30, per an AFP wire report, and won just 41% of his second-serve points over the match's final three sets. He said he plans to "run some tests to see what comes out of it," leaving the cause of the dizziness and vision disturbances unexplained for now.

"DIZZY AND SEEING SPOTS AND COULDN'T RECOVER" — Félix Auger-Aliassime

Khachanov, a former top-10 player and 2022 US Open semifinalist now ranked No. 50, picked up his first top-10 win of the season with the result and struck a sympathetic note about his opponent's condition. "I wish him to feel better and speedy recovery if he is injured," he said. Khachanov advances to face either Ignacio Buse of Peru or France's Benjamin Bonzi in the third round.

The loss comes with baggage attached: Auger-Aliassime had already withdrawn from last month's Canadian Open with a back injury, adding to concern about his physical readiness heading into the fall hard-court stretch. His early exit is expected to knock him from his current No. 4 ATP ranking when the new rankings post.

What's next: Auger-Aliassime said he'll undergo medical testing to identify the cause of the dizziness before his next tournament. Khachanov continues his run at the Open, with the winner of his third-round match advancing deeper into a draw that concludes with the men's final on September 13.