World number one and WGC-Cadillac Match Play top seed Rory McIlroy has been drawn in a difficult group with three Americans at this week's tournament at TPC Harding Park in San Francisco.

The Northern Irishman is joined in group one by 2014 FedEx Cup champion Billy Horschel, 2012 FedEx Cup winner Brandt Snedeker and 2013 US PGA Champion Jason Dufner.

The tournament has a new format this year, with 16 groups of four and one winner from each group advancing to the knockout stages after the round-robin games between Wednesday and Friday.

Group two contains recent Masters winner Jordan Spieth and Lee Westwood, while elsewhere Bubba Watson has been drawn in a heavyweight group four with Louis Oosthuizen, Keegan Bradley and Miguel Angel Jimenez.

WGC-Cadillac Match Play groups

Group 1: Rory McIlroy, Billy Horschel, Brandt Snedeker, Jason Dufner
Group 2: Jordan Spieth, Lee Westwood, Matt Every, Mikko Ilonen
Group 3: Henrik Stenson, Bill Haas, Brendon Todd, John Senden
Group 4: Bubba Watson, Louis Oosthuizen, Keegan Bradley, Miguel Angel Jimenez
Group 5: Jim Furyk, Martin Kaymer, Thongchai Jaidee, George Coetzee
Group 6: Justin Rose, Ryan Palmer, Anirban Lahiri, Marc Leishman
Group 7: Jason Day, Zach Johnson, Branden Grace, Charley Hoffman
Group 8: Dustin Johnson, Victor Dubuisson, Charl Schwartzel, Matt Jones
Group 9: Adam Scott, Chris Kirk, Paul Casey, Francesco Molinari
Group 10: Sergio Garcia, Jamie Donaldson, Bernd Wiesberger, Tommy Fleetwood
Group 11: Jimmy Walker, Ian Poulter, Webb Simpson, Gary Woodland
Group 12: J.B. Holmes, Brooks Koepka, Russell Henley, Marc Warren
Group 13: Rickie Fowler, Graeme McDowell, Shane Lowry, Harris English
Group 14: Matt Kuchar, Hunter Mahan, Stephen Gallacher, Ben Martin
Group 15: Patrick Reed, Ryan Moore, Danny Willett, Andy Sullivan
Group 16: Hideki Matsuyama, Kevin Na, Joost Luiten, Alexander Levy