Søren Kjeldsen prevailed from a classic encounter to beat World Number Two Rory McIlroy 2 and 1 on day one of the WGC - Dell Technologies Match Play at Austin Country Club.
McIlroy - the 2015 champion - was one of the favourites coming into the week but will now face an uphill battle to get out of Group 2 after Kjeldsen produced a stunning performance.
The Dane finished with four birdies to win a roller coaster of a contest where the lead changed hands five times.
Kjeldsen put his approach to the first to six feet to quickly go one up with a birdie but McIlroy hit straight back on the third with the help of a huge drive and a 13-foot putt.
A tee-shot to 16 feet on the par three fourth moved the Northern Irishman one up but Kjeldsen was displaying some wonderful iron play and an approach to ten feet on the fifth brought a second birdie of the day.
A McIlroy bogey on the ninth then handed the Dane a one-shot lead at the turn and a birdie on the tenth extended that advantage.
A hat-trick of birdies soon wiped that out, however, as McIlroy put his tee-shot close on the 11th, got on the par five 12th in two and almost drove the green at the next.
But Kjeldsen put his approaches to the 14th and 16th to inside ten feet and then left a tap-in on the par three 17th to claim victory.
In the other match in Group 2, Gary Woodland beat Emiliano Grillo 3 and 2.
In Group 15, Branden Grace claimed an ultimately comfortable 4 and 2 win over Andy Sullivan.
Sullivan was ragged off the tee on the third to send Grace one up but his tee-shot on the fourth was to within ten feet to even things up.
Grace hit an approach close on the fourth to edge back ahead and then holed a brilliant bunker shot on the seventh to establish a cushion before Sullivan hit back with a birdie on the next.
An approach to six feet on the 11th then re-established the South African's advantage and when Sullivan found the water on the 12th, he was three up.
Grace went dormie four with an approach to tap-in range on the 14th and while Sullivan won the next, he could not complete a fightback.
In the other Group 15 encounter, William McGirt beat Brandt Snedeker two up.
