Nick Kyrgios beat defending champion Daniil Medvedev 7-6(11), 3-6, 6-3, 6-2 at the US Open on Sunday.
The 23rd seed had a 3-1 ATP Head2Head lead over Medvedev and was in great form, but beating the top seed at a Grand Slam was a new challenge.
Australian passed the test with flying colours, playing his aggressive game to triumph after two hours and 53 minutes."What a place to do it," Kyrgios said. "Packed house in New York!"
Medvedev will lose his top spot in the Pepperstone ATP Rankings on the Monday after the US Open.
Rafael Nadal, Carlos Alcaraz or Casper Ruud will become World No. 1. Kyrgios advances to his first quarter-final in New York, where he will next play Karen Khachanov.
"I want to go all the way," said Kyrgios, whose victory was his third against a World No. 1. "Hopefully it is possible."
The match was less about Medvedev playing poorly — he made just 19 unforced errors — and more about Kyrgios shining under the spotlight.
The fan favourite fired 21 aces and 53 total winners. He won 62% (29/47) of his net points against Medvedev.
"I just played well. I returned unbelievable today. Third and fourth sets were so free "KYRGOS: "I enjoyed every moment on Ashe today. Thrilled."
Kyrgios emerged victorious despite a third-set gaffe that would have thrown many players for a loop.
The 27-year-old cost himself a break point opportunity at 30-all when he ran around the net to hit on the full a Medvedev floating volley that clearly lacked the velocity to make it back into play.
It was for pure comic relief, but the chair umpire correctly awarded the point to Medvedev, who would serve out the game.
Instead of letting the mistake get to him, Kyrgios remained calm and found his best tennis of the tournament to surge past the top seed.