Phil Mickelson struggles at Oakmont CC ahead of his attempt to complete career Grand Slam at the US Open

Oakmont Country Club is a notoriously challenging golf course, something Phil Mickelson has now been reminded of. The golfer struggled mightily in previews ahead of the US Open this weekend.

Should Mickelson come out victorious, which is a long shot that seems a little more difficult given his practice results, he would become the seventh player ever to complete a career Grand Slam.

The rough has been particularly brutal to players at Oakmont. Mickelson is just the latest player to fall victim, but he struggled with it on Wednesday (June 11). His short game has always been his strength, but even that was no match for the tall grass off the green.

From just a few feet off the green, Mickelson's first chip attempt moved the ball a few inches. Subsequent chips were better, but he still left the ball pretty short on the green.

Rory McIlroy completed his Slam earlier this year, and Mickelson has needed the US Open title to complete it since 2013 when he won the Open Championship. Mickelson has an incomprehensible six runner-up finishes in this event, so he's been as close as one stroke, which he fell short by in 1999 to Payne Stewart and in 2006 to Geoff Ogilvy.

Mickelson is considered a longshot at Oakmont this week. Per CBS Sports, he's +30000 to win, which is a far cry from Scottie Scheffler (+280) at the top. However, Mickelson is in the middle of the pack overall, so he's not the longest shot.

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US Open favorite says Oakmont golf course is no joke

Scottie Scheffler is the favorite to win the US Open at Oakmont Country Club. If he does, it will be after a truly difficult four rounds. Ahead of the tournament's official tee-off tomorrow morning, Scheffler said Oakmont is exceptionally difficult.

Scottie Scheffler said the course is hard (Image via Imagn)

Via Bleacher Report, Scheffler said:

"This is probably the hardest golf course that we'll play. Maybe ever."

That's a bold statement from the world's top-ranked golfer right now. He added that it's a whole lot harder than playing Augusta National, where he has two wins:

"When you miss the green at the Masters, the ball runs away and it goes into these areas, and you can play a bump, you can play a flop. There's different options. Here, when you hit the ball over the green, you just get in some heavy rough, and it's like, 'Let me see how I can pop the ball out of this rough and somehow give myself a look.'"

The infamous rough has played a role in practice, and Scheffler is ready for it to impact the tournament in ways that would frustrate the field.