Rahm birdied the final two holes at Torrey Pines last year to snatch victory by a single shot from Louis Oosthuizen, but the Ryder Cup star learnt recently that his closing 67 may not have been quite as good as he thought it was.
"It's easy to think you need to be playing perfect golf," Rahm recalled before teeing off at Brookline at 7.18am local time (12.18 Irish time) alongside Open champion Collin Morikawa and 2021 US Amateur winner James Piot.
"I remember watching my highlights of Sunday last year, and I thought I played one of the best rounds of my life, and I kept thinking I cannot believe how many fairway bunkers I hit that day, how many greens I missed and how many putts I missed.
"You know, it's golf, and that's how it is. You truly don't have to play perfect, and that's I think the best lesson I can take from that.
"There's no extra pressure (as defending champion). It's pretty much the same as it's always been with the different factor being that I've already won a major so I feel like a lot of the pressure I used to put on myself is not really there.
"I feel like I can enjoy it a little bit more and know that you don't need to do anything special to get it done."
Which other players are making headlines?
Rory McIlroy has been made favourite to win his first major title since 2014 on the back of his RBC Canadian Open victory on Sunday, but most eyes will be on Phil Mickelson as he celebrates his 52nd birthday during today's first round.
Mickelson has become the figurehead of the Saudi-backed LIV Golf series which is causing a split in the game and the reaction of the vocal Boston fans will be fascinating.
Key tee times (all Irish time) 1218 Jon Rahm, Collin Morikawa, James Piot 1240 Rory McIlroy, Hideki Matsuyama, Xander Schauffele 1814 Justin Thomas, Viktor Hovland, Tony Finau 1836 Matt Fitzpatrick, Webb Simpson, Dustin Johnson 1847 Phil Mickelson, Shane Lowry, Louis Oosthuizen
Weather forecast Cool and mostly clear overnight into this morning with the low temperature nearing the mid 50s (11-14 celsius). Cloud cover looks to build back in for today while south to southwest winds offer warmer temperatures on Friday. Winds still capable of gusting over 20 mph (32 kph) at times will transition west and north for the weekend, with cooler and drier air settling back in.