FORT WORTH, Texas — Jason Bohn, Jeff Overton and Blake Adams shot 7-under 63s on Thursday to share the first-round lead at the Colonial, where 2008 champion Phil Mickelson's return turned into a struggle after an impressive start.

Mickelson birdied the first two holes, but that was the lowest he got on a hot day with ideal scoring conditions. Lefty finished with a 1-over 71 and is tied for 89th place, while 75 players were under par.

It will take quite a comeback for Mickelson to win his third Colonial, a victory that would push him to No. 1 in the world ranking ahead of Tiger Woods for the first time in his career. He has work to do just to make the cut and participate in the tournament's second "Pink Out" during the third round Saturday.

Mickelson wasn't at a pink-swathed "Hogan's Alley" to defend his title last May after finding out that his wife had breast cancer. That is when the Colonial had its first "Pink Out" to honor Amy Mickelson and raise awareness of the disease.

Overton shot his career-best round after being even through seven holes, while Bohn, who won last month in New Orleans, had his season-best round despite congestion so bad that he couldn't hear out of his left ear. Adams was in the last group of the day.

After Mickelson blasted from a greenside bunker to 10 feet for a birdie at the opening par-5, 565-yard hole, he put his drive at the dogleg-right 389-yard 2nd hole only 50 yards from the pin and pitched to 8 feet for another birdie.

Walking off the No. 3 tee box after another impressive drive, Mickelson was flashing the thumbs-up sign and acknowledging the verbal kudos coming from spectators in the gallery.

By the time he knocked in a bogey putt at the 18th hole, Mickelson was shaking his head trying to figure out what happened. The Masters champion hit only five of 14 fairways and missed six greens.

"There's no excuse. There's no wind, the golf course is in perfect shape, and there were a lot of scores out there," Mickelson said. "I just wasn't one of them."

Brian Davis was alone in fourth after shooting 64 despite a swollen face from an abscess inside his mouth. Davis left after his round to go see a doctor.

There was a group of nine players at 65, one shot better than John Daly, who had his first bogey-free round in two years. Defending champion Steve Stricker shot 68.

Mickelson managed to save par at the 406-yard 6th hole despite sending his drive into the right rough and then hitting his approach only 37 yards — 43 yards away from the pin and still in the rough.

The next tee shot ricocheted backward off a tree, seemingly a break since he had a clear line to the No. 7 green despite being in the right rough. But Mickelson pushed his approach well left of the green into a hazard, forcing him to take a penalty on his way to double bogey. His only other birdie came on a 16-foot putt at the par-3 8th.

"I played well the first four holes, then the last 14, I hit the ball terrible, so I'll have some work to do," Mickelson said. "Already was in there texting Butch (Harmon, his coach) and so I've got some direction. In the morning, I'll probably get out here early."

Mickelson bogeyed two of the last three holes, saving par at No. 17 despite his approach shot hitting a television tower.

Bohn woke up around 1 a.m. with sharp shooting pains in his left ear and so much congestion that he didn't sleep much the rest of the night.

"But it didn't affect my play too much, so I'm not really complaining," said Bohn, who planned to see a doctor Thursday afternoon.

A week ago at the Byron Nelson Championship in Irving, about 25 miles away, Jason Day showed up before the first round feeling ill after a reaction to antibiotics that were supposed to wipe out a sinus infection. He thought about withdrawing and going home to Fort Worth, but shot an opening 4-under 66 and went on to get his first PGA Tour victory.

Maybe that's a good omen for Bohn, who a month ago won the Zurich Classic for his first victory in five years.

"That would be one heck of a coincidence if that happened," he said. "Two Jasons and two illnesses."

In the first group of the day off No. 1, Overton was even until his 7-iron tee shot on the 194-yard 8th hole landed 6 feet from the pin. That set up the first of seven birdies in a 10-hole stretch. His backside 6-under 29 that was one off the course record.