Søren Kjeldsen rediscovered his form in spectacular fashion to defeat World Number Two Rory McIlroy on day one of the WGC - Dell Technologies Match Play at Austin Country Club.
The Dane was playing just his second European Tour event of the season and by his own admittance had failed to find his best golf in 2017, with four-time Major Championship winner McIlroy presenting a formidable challenge.
Kjeldsen finished with four birdies to win a roller coaster of a contest where the lead changed hands five times.
"That was a good day," said Kjeldsen. "He's such a great player, so I knew I had to go and play well. I had to focus on what I was doing and I played really well today.
"My mindset was if I go and shoot a good score, he has to play well to beat me. I obviously know he's capable of that but I managed just to squeeze it in."
Kjeldsen put his approach to the first to six feet to quickly go one up with a birdie but McIlroy - courtesy of back-to-back birdies - took the lead at the fourth before the Dane levelled the scores back up with a gain at the next.
A McIlroy bogey on the ninth then handed Kjeldsen a one-shot lead at the turn and a birdie on the tenth extended that advantage before the Northern Irishman's hat-trick of gains put him one up after 13.
But Kjeldsen put his approaches to the 14th and 16th to inside ten feet for birdies and then left a tap-in gain on the par three 17th to claim victory.
In the other match in Group 2, Gary Woodland beat Emiliano Grillo 3 and 2.
Tyrrell Hatton fought back to secure an impressive 2 and 1 win over Charles Howell III in their Group 10 match-up.
He's such a great player, so I knew I had to go and play well - Søren Kjeldsen
The American opened up with a birdie on the first to go one up before Hatton levelled on the fourth, but back-to-back Howell III gains on the fifth and sixth put him two up.
Hatton responded with a par to reduce the score to one shot on the seventh and brought it back to all square with a birdie on the tenth before the Englishman took the lead for the first time on the next after Howell III found water.
A Hatton blemish restored parity on the 12th but successive bogeys on 13 and 14 from the American handed the World Number 14 an advantage he could not claw back.
Hatton said: “I'm really happy to get my first win. Obviously it's my first time here. I will take my competition tomorrow and hopefully I can play better.”
Rafa Cabrera Bello continued his excellent match play form to earn a routine 4 and 3 success over Jeunghun Wang in the other Group 10 opener.
The Spaniard, who reached the semi-final in last year's competition, went one up at the second after Wang went out of bounds with his second shot before a par-birdie on the fourth and fifth put Cabrera Bello into a three up lead.
A bogey from Cabrera Bello on the sixth and a Wang birdie reduced the score to one up at the turn but a gain on the 11th restored a two up lead for the 32 year old before consecutive blemishes from the South Korean on the 14th and 15th sealed the victory for the World Number 26.
“I have been really looking forward to it. Pretty much since last year really, I wanted to come back and play,” Cabrera Bello said.
Sergio Garcia and Shane Lowry halved an enthralling Group 7 encounter which climaxed with the Irishman missing a six-foot putt on the final hole for victory.
The duo traded bogeys on the opening two holes before another blemish from Lowry on the fourth and a birdie from the Spaniard on the fifth put Garcia two up.
A Lowry birdie on the sixth and a Garcia blemish on the eighth brought the scores back to all square before the 29 year old hit consecutive gains on the ninth and tenth to go two up.
Lowry hit another bogey on the 12th before the dramatic finale on the 18th - which means both players still remain in contention for the one knockout place.
Jon Rahm secured a 2 and 1 success over Kevin Chappell to sit on top of Group 7 after the first round of matches.
Chappell carded a birdie-bogey start to keep the scores level before the Spaniard sank an 18-foot putt to go one up after the fourth before extending his lead at the eighth following a blemish from the American.
Successive birdies from Chappell after the turn brought the scores back to all square but gains from Rahm at the 12th, 15th and 16th surged the Spaniard to an opening day win.
Rahm said: “I feel as a Spaniard, I love match play, and we all do, and normally we're good at it. I had a lot of fun. I played a lot of good golf and barely missed any shots.”
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 but birdies on the fifth and seventh put the South African two up.
Sullivan hit back with a birdie of his own on the eighth to reduce the score before Grace was soon three up following a gain on the 11th and the Englishman finding water on the 12th.
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.
