Graeme McDowell and Shane Lowry have been drawn in the same group for the WGC Matchplay Championship, which begins on Wednesday in  San Francisco

The two Irishmen are in Group 13, along with Rickie Fowler and Harris English.

Top seed and four-time major winner Rory McIlroy has been handed a tough draw after being grouped with Billy Horschel, Brandt Snedeker and Jason Dufner.

It is a group stacked with winners, as Horschel was the 2014 FedEx Cup Champion, Snedeker won the Fed Ex Cup in 2012, and Dufner won the 2013 PGA Championship.

Under the tournament's new format, the current top 16 in the world rankings were each placed in a different group, and the remaining three players were drawn from pools depending on their current rankings.

There will be round-robin matches between the four players in the group between Wednesday and Friday, with the winners advancing to face off in a World Cup-style format.

McIlroy will start against Dufner on Wednesday, and will hope to be able to advance to face the winner of Group 16, headlined by Hideki Matsuyama and also featuring Kevin Na, Joost Luiten and Alexander Levy.

Masters winner Jordan Spieth, ranked second in the world, has been paired with Lee Westwood, Matt Every and Mikko Ilonen - who was the last player in the field of 64 after Phil Mickelson's late withdrawal for personal reasons.

While Westwood has a long record of success in match play, Spieth can boast two wins and two second places in five starts.

Justin Rose, sixth in the world following his victory at the Zurich Classic in New Orleans on Sunday, will be up against Ryan Palmer, Anirban Lahiri and Marc Leishman.

Although Leishman finished in a tie for 28th this weekend, he like Rose will have come away from TPC Louisiana with a good deal of confidence following a second-round 63.

Group 4 also threw up plenty of intrigue with Bubba Watson paired with Louis Oosthuizen, Keegan Bradley and Miguel Angel Jimenez.

Ian Poulter, who has a 22-12 record in match play, is in Jimmy Walker's group along with Webb Simpson and Gary Woodland, while Paul Casey, 16-9 in match-play, is in Adam Scott's group with Chris Kirk and Francesco Molinari.