In a match that saw the opening six holes being exchanged at Austin Country Club, Morikawa always looked to be in control and, despite chipping into the water on the back nine after MacIntyre had already been wet off the tee, the two-time major winner registered a 2&1 victory.
It was MacIntyre’s first defeat in the round-robin stage of the event, having opened with a win over Kevin Na on his debut last year before then halving with both Dustin Johnson, who was the world No 1 at the time, and Adam Long.
His start on this occasion left the Scot needing to win both of his remaining matches against American Jason Kokrak and Spaniard Sergio Garcia to have any chance of progressing to the knockout stage again.
Having hit just four of the opening 10 greens in regulation, MacIntyre found himself two down to world No 2 Morikawa before the hole he was in became deeper as a wayward tee shot allowed his opponent to win the 12th through a concession.
The left-hander from Oban was then left off the hook as Morikawa sent that chip racing past the hole and into the water at the 13th, which was halved in bogey-5s.
After hitting one of his best shots of the day, MacIntyre was unable to convert a golden birdie chance at the par-4 14th before making amends by rolling in a 22-footer for a birdie at the next.
But, unlike Long in his match against him last year, Morikawa wasn’t going to let MacIntyre reel him in, closing the door with two straightforward pars in a match involving players who were on opposite sides in the 2017 Walker Cup in Los Angeles.
“Luck,” said Morikawa in reply to being asked what had been the key to his win. “I told myself to wake up the past couple days to get ready for this and I just did everything I told myself not to do. I made a few mistakes out there.”
In the other match in their group, Garcia beat Kokrak 4&3 as the Spaniard clocked up his 29th victory in the event.
Other opening-day winners included world No 3 Viktor Hovland, local man Jordan Spieth and former Masters champion Adam Scott.
“It was very up and down,” said Hovland of his one-hole win over Austrian Sepp Straka. “It wasn't pretty but, making seven birdies, it was nice to get the win.”
Spieth had to wait until the 16th hole to get his nose in front for the first time against fellow American Keegan Bradley before going on to win by two holes.
“The course is designed for the south wind, so when you get the north wind it's very tricky,” said Spieth of players finding the conditions troublesome.
In the same group, Scott beat Justin Rose by two holes. “It was a real pillow fight,” said Scott of that match. “We gave each other a lot of holes, certainly nothing for me to brag about.”
Later in the day, defending champion Billy Horschel opened with a win, as did English duo Matt Fitzpatrick and Tyrrell Hatton and Irishman Seamus Power, who needs to finish in the top two in his group to secure a Masters debut next month.
On his return from a hand injury, Bryson DeChambeau was held by British Masters champion Richard Bland but another Englishman, Paul Casey, conceded his match against Canadian Corey Conners after just two holes due to back spasms.
Full first-day results (seeding in brackets)
Jon Rahm (1) bt Sebastian Munoz (58) 4&2
Patrick Reed (23) lost to Cameron Young (40) 1 hole
Collin Morikawa (2) bt Robert MacIntyre (61) 2&1
Jason Kokrak (22) lost to Sergio Garcia (43) 4&3
Viktor Hovland (3) bt Sepp Straka (63) 1 hole
Will Zalatoris (24) bt Cameron Tringale (45) 5&4
Patrick Cantlay (4) halved with Keith Mitchell (62)
Sungjae Im (21) lost to Seamus Power (42) 5&4
Scottie Scheffler (5) bt Ian Poulter (59) 2&1
Matt Fitzpatrick (20) bt Tommy Fleetwood (41) 1 hole
Justin Thomas (6) lost to Luke List (53) 3&2
Kevin Kisner (29) bt Marc Leishman (37) 4&3
Xander Schauffele (7) bt Takumi Kanaya (56) 3&2
Tony Finau (18) lost to Lucas Herbert (39) 4&3
Dustin Johnson (8) bt Mackenzie Hughes (51) 3&2
Max Homa (30) bt Matthew Wolff (38) 3&1
Bryson DeChambeau (9) halved with Richard Bland (54)
Talor Gooch (27) bt Lee Westwood (47) 3&2
Louis Oosthuizen (10) lost to Alex Noren (50) 1 hole
Paul Casey (19) conceded to Corey Conners (36)
Jordan Spieth (11) bt Keegan Bradley (60) 2 holes
Adam Scott (32) bt Justin Rose (46) 2 holes
Billy Horschel (12) bt Min Woo Lee (49) 3&2
Thomas Pieters (26) bt Tom Hoge (33) 2&1
Tyrrell Hatton (13) bt Christiaan Bezuidenhout (52) 3&2
Daniel Berger (17) lost to Si Woo Kim (48) 2 holes
Joaquin Niemann (14) lost to Maverick McNealy (64) 8&6
Kevin Na (25) bt Russell Henley (34) 1 hole
Abraham Ancer (15) bt Bubba Watson (57) 3&1
Webb Simpson (31) bt Brian Harman (44) 1 hole
Brooks Koepka (16) bt Erik van Rooyen (55) 3&2
Shane Lowry (28) lost to Harold Varner III (35) 2&1
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