Masters champion Jordan Spieth and world number one Rory McIlroy enjoyed easy victories on the first day of the World Golf Championships Match Play.
Spieth, 21, defeated Finland's Mikko Ilonen 4&2 while McIlroy beat American Jason Dufner 5&4 in San Francisco.
But Jason Day, Justin Rose, Ian Poulter, Adam Scott and Graeme McDowell all lost at Harding Park.
"I don't feel very hopeful right now," said Rose of his chances of progressing from his round-robin group.
The world's top 16 are the leading seeds in as many groups of four players, with the winners progressing to the knockout stage.
Rose lost to Australia's Marc Leishman, who won the opening hole with a birdie and went on to seal a 3&2 win.
Poulter also lost the opening hole to a birdie from former US Open champion Webb Simpson but four bogeys in six holes were ultimately his downfall.
The Englishman later tweeted: "Truly disgusting putting today, it was actually farcical. Spent three hours on the putting green this afternoon. We will see if it helped."
Lee Westwood continued his good form after winning in Jakarta on Sunday, beating Matt Every on the 18th, while fellow Englishman Danny Willett enjoyed a successful debut in the event with a 3&2 win over Ryan Moore.
Group 1: 1-Rory McIlroy (NI), 18-Billy Horschel (USA), 35-Brandt Snedeker (USA), 53-Jason Dufner (USA)
Group 2: 2-Jordan Spieth (USA), 26-Lee Westwood (Eng), 40-Matt Every (USA), 62-Mikko Ilonen (Fin)
Group 3: 3-Henrik Stenson (Swe), 23-Bill Haas (USA), 42-Brendon Todd (USA), 60-John Senden (Aus)
Group 4: 4-Bubba Watson (USA), 29-Louis Oosthuizen (RSA), 33-Keegan Bradley (USA), 63-Miguel Angel Jimenez (Spa)
Group 5: 5-Jim Furyk (USA), 17-Martin Kaymer (Ger), 44-Thongchai Jaidee (Tha), 59-George Coetzee (RSA)
Group 6: 6-Justin Rose (Eng), 22-Ryan Palmer (USA), 34-Anirban Lahiri (Ind), 56-Marc Leishman (Aus)
Group 10: 10-Sergio Garcia (Spa), 30-Jamie Donaldson (Wal), 39-Bernd Wiesberger (Aut), 54-Tommy Fleetwood (Eng)
Group 11: 11-Jimmy Walker (USA), 27-Ian Poulter (Eng), 46-Webb Simpson (USA), 50-Gary Woodland (USA)
Group 13: 13-Rickie Fowler (USA), 32-Graeme McDowell (NI), 47-Shane Lowry (Ire), 55-Harris English (USA)
Group 14: 14-Matt Kuchar (USA), 31-Hunter Mahan (USA), 41-Stephen Gallacher (Sco), 61-Ben Martin (USA)
Group 15: 15-Patrick Reed (USA), 29-Ryan Moore (USA), 49-Danny Willett (Eng), 61-Andy Sullivan (Eng)
Group 16: 16-Hideki Matsuyama (Jap), 21-Kevin Na (USA), 44-Joost Luiten (Ned), 54-Alexander Levy (Fra)