Olympic humbles Mickelson, Watson
Mickelson was in trouble from his first tee shot, at the par-four ninth. It got lost in a tree and he went back to the tee.
He did well to salvage a bogey, but things didn't improve.
"I didn't play well, obviously," said Mickelson, who has won four major titles but never nabbed a US Open trophy.
He has a record five runner-up finishes in the event, but Thursday's round left him hoping he could just make the cut on Friday.
"I fought hard there for a while trying to keep it a few over," said Mickelson, who finished with a six-over 76.
"Three-putting four really hurt because I probably tried to get a little aggressive. I felt like I needed one birdie there and I ended up three-putting.
"It was a tough day when you play the way I did."
Watson, who claimed his first major title at The Masters in April, posted an eight-over 78.
"It beat me up today," he said of the 7,170-yard Olympic layout.
Watson had said before the tournament he wasn't sure if he liked Olympic, and he didn't sound as if he'd been won over after a round that included seven bogeys and a double-bogey and just one birdie.
"It's disappointing starting off like this, doesn't matter what tournament," Watson said. "Every year we're going to play a different US Open venue, so I've got to just come out and try to play it."
Watson's double-bogey came at the par-four 18th, his 10th hole of the day.
"Club on the chip got twisted and I got an awkward spot there and missed the putt," he said. "So quick double-bogey there, quick five-over.
"Even with a birdie, just couldn't get anything going. Never got any rhythm."
Mickelson said he thought he made some beneficial adjustments around the turn, and he hoped for more improvements on Friday.
"The score didn't really stay great, but I started to hit it a little bit better," he said. "I've got a tough challenge just to get to the weekend tomorrow, unfortunately.
"I'll go out tomorrow and see if I can shoot something under par," added Mickelson, saying he'd try to "put this round aside for now, and see if I can just play one round under par.
"Maybe that will get me to the weekend."
Woods, who stayed steady to post a one-under 69 that left him three shots off Michael Thompson's early lead, said it didn't take much for things to go horribly wrong on the unforgiving Olympic course.
"This golf course, it's so demanding," Woods said. "If you're off your game just a little bit, you're going to pay the price. Phil and Bubba were off just a little bit."