SAN DIEGO, Calif., June 15 (Reuters) - Bryson DeChambeau will begin his U.S. Open title defence in a group with reigning Masters champion Hideki Matsuyama and 2020 U.S. Amateur champion Tyler Strafaci during first-round action on Thursday at Torrey Pines. DeChambeau, who won his first major title at last year"s U.S. Open by a stunning six shots at Winged Foot, will tee off from the par-four first hole at 1:14 p.m. PT (2014 GMT) with Japan"s Matsuyama and Strafaci. Six-times runner-up Phil Mickelson, fresh off a historic win at the PGA Championship and looking to complete the career Grand Slam on his home course, will start at 10:51 a.m. from the par-four 10th hole with Xander Schauffele and Max Homa. In other groupings, world number one Dustin Johnson will set off from the first hole with Northern Irishman Rory McIlroy and Englishman Justin Rose, while tournament favourite Jon Rahm will start on the back nine with former Masters champion Patrick Reed and Australian Marc Leishman. For world number three Rahm, this will mark his first start since he was forced to withdraw from the Memorial Tournament with a commanding six-shot lead after 54 holes because of a positive COVID-19 test. read more Rahm, in search of his first career major, will be in comfortable surroundings on the South Course at Torrey Pines as he has four top-10 finishes in five events played there. Brooks Koepka, who counts two U.S. Open wins among his four majors and finished runner-up at the PGA Championship, will go out in the fifth group off the 10th tee at 10:29 a.m. with world number two Justin Thomas and No. 4 Collin Morikawa. First-round action will begin at 9:45 a.m. and after two rounds the 156-player field will be reduced to the low 60 scores and ties. Reporting by Andrew Both; Writing by Frank Pingue in Toronto Editing by Christian Radnedge