Rory McIlroy lost his opening match against Peter Uihlein at the World Golf Championships in Austin.
The 28-year-old, who won his first tournament in 18 months at the Arnold Palmer Invitational last week, was beaten 2&1 in the opening group game.
Ian Poulter secured a 3&2 victory over fellow Englishman Tommy Fleetwood, while defending champion Dustin Johnson lost 3&1 against Bernd Wiesberger.
The matchplay competition features 16 groups, each made up of four players.
England’s Tyrrell Hatton won his first match, beating Frenchman Alexander Levy 3&2, however Hatton’s compatriot Ross Fisher lost 2&1 to Japan’s Yuta Ikeda.
McIlroy faces an uphill struggle to reach the last-16 as wins over Jhonattan Vegas on Thursday and Brian Harman on Friday may not be enough if Uihlein goes on to win all three group matches.
World number two Justin Thomas avoided an embarrassing defeat to Luke List, who was forced to putt with a wedge after bending his putter out of shape in frustration during the early stages.
“I was walking off the sixth green and I was a little unhappy about the way I was feeling, a little under the weather,” List said.
“I thought it was a brush area and I just kind of swiped my putter and it turned out to be a wall. Stupid on my part.”