The American, competing a week after making his debut on the Saudi-backed LIV circuit, carded a three-over-par 73 at The Country Club outside Boston which brought him to 11-over on the week and well outside the cut-line.

"I enjoyed the week. Wish I had played better," said Mickelson.

"The U.S. Open is the ultimate test and you don't really know where your game is until you get tested.

"I thought I was a little bit closer than I was, but I really struggled putting. I thought I was more prepared than I was."

The six-time major champion has faced plenty of criticism over his involvement with LIV Golf, funded by Saudi Arabia's Public Investment Fund and fronted by Australian Greg Norman.

He's also taken flak for comments he made about Saudi Arabia, which led to him taking a hiatus from golf.

"I thought I was a little bit closer than I was, but I really struggled putting. I thought I was more prepared than I was." - Phil Mickelson.

That decision came in February when excerpts from an unauthorised biography on Mickelson revealed the golfer called the Saudis "scary" but that he would look past their human rights records to gain leverage with the PGA Tour.

"I missed competing, but I also enjoyed some time away," Mickelson said after his round.

Mickelson, who has lost several sponsors since his comments surfaced, apologised but still faced an onslaught of questions from the media when he arrived on site earlier this week.

But the long-time fan favourite was given a warm reception from spectators at The Country Club, flashing a thumbs-up as he walked the fairways to shouts of "we love you" and "let's go Phil" from onlookers.

"The fans here have always been terrific, and they really support all sports, and I love it when we bring golf here because they create a really special atmosphere," Mickelson said.

Mickelson effectively ended his latest bid to complete the career grand slam of golf's four major titles when he opened the year's third major with an eight-over par 78 on Thursday.

In Friday's second round, he played better but it was far from enough to undo the damage already done.

Mickelson bogeyed three consecutive holes starting at the par-4 13th, his fourth hole of the day, and offset two birdies with a pair of bogeys over a late four-hole stretch.

When he walked off the course, Mickelson was 143rd among 156 players in the field.

Mickelson, one of 17 players suspended by the PGA Tour for joining LIV Golf, is expected to play again later this month when the breakaway circuit resumes action in Portland.