Rory McIlroy lost to world number 68 Soren Kjeldsen in his opening group match at the World Golf Championship Match Play in Austin, USA.
McIlroy, the world number two, went down 2&1 as Denmark's Kjeldsen sank his fourth birdie in a row on the 17th.
The Northern Irishman must beat Gary Woodland and Emiliano Grillo in his other Group Three matches to have a chance of reaching the last 16.
There were also defeats for Jordan Spieth and Danny Willett.
Two-time major winner Spieth won just one hole before losing 4&2 to Japan's Hideto Tanihara, while Masters champion Willett's recent struggles continued as he lost to Korea's KT Kim by the same score.
World number three Jason Day withdrew after six holes of his match against American Pat Perez to be with his ill mother.
Only one player who lost their first game 12 months ago - current world number one Dustin Johnson - went on to reach the last 16, with Johnson having no such problems this year after a 5&3 win over Webb Simpson.
There were victories for England's Tommy Fleetwood over Zach Johnson and Paul Casey against Joost Luiten and for the in-form Tyrrell Hatton.
Hatton was two down to American Charles Howell III after six holes but recovered over the back nine to win 2&1.
But Ross Fisher, Matt Fitzpatrick and Andy Sullivan suffered heavy defeats to Louis Oosthuizen, Kevin Na and Branden Grace respectively,
Lee Westwood was also an opening-day loser after slumping to a 3&2 loss against Australian Marc Leishman.
However, Scotland's Russell Knox was a 3&2 winner over American Bill Haas.