Miami - Rory McIlroy and Jordan Spieth suffered upset defeats as the underdogs bared their teeth on the opening day of the World Golf Championships-Dell Match Play on Wednesday.
McIlroy's quest to regain his match play crown was dealt an early setback as the world number two crashed to a 2&1 defeat against Danish veteran Soren Kjeldsen at Austin Country Club in Texas.
An out-of-form world number six Spieth was then comprehensively beaten 4&2 by Japan's Hideto Tanihara.
Match play specialist McIlroy, the 2015 champion, had clawed his way back from two down to make it all square heading into the final three holes.
But McIlroy missed a difficult birdie putt on the 16th to halve the hole, leaving Kjeldsen one up with two to play.
The 41-year-old Dane then produced a magnificent tee-shot to within three feet on the par-three 17th to pile the pressure on McIlroy.
The Northern Irishman made the green with his tee shot but was unable to sink his birdie, leaving Kjeldsen with the simplest of tap-ins to complete a 2&1 victory.
"I felt good coming into today," Kjeldsen said. "He's a pretty spectacular player so you don't want to watch him too much.
"I knew I had to play well to have any kind of a chance so I didn't look at what Rory was doing and fortunately I played well," added Kjeldsen, who as the world number 68 is one of the lowest-ranked players in the in the 64-man field.
"Everyone I'm playing this week I'm going to be the underdog," Kjeldsen joked after the Group 2 win.
"It's nice. I haven't had the best start to the season in America. But the last few weeks I've been getting better. I feel good. You can talk about match play but at the end of the day you need to shoot good scores and that's what I did today."
This week's tournament sees the field split into 16 groups of four players who play each other on a round robin basis through Friday. The 16 group winners then advance to the knockout rounds starting on Saturday.
In the day's other Group 2 game, Gary Woodland of the United States defeated Argentina's Emiliano Grillo 3&2.
McIlroy will attempt to reignite his challenge on Thursday against Woodland.
England's Paul Casey defeated Joost Luiten of the Netherlands 2&1 in Group 12 where South Africa's Charl Schwartzel thrashed An Byeong-Hun of South Korea 6&5.