4 takeaways from the opening round at U.S. Open

Justin Rose tops a loaded leaderboard after Round 1 of the U.S. Open. Rickie Fowler, Xander Schauffele, and a plethora of other marquee players are chasing the Englishman with 54 holes remaining, setting the stage for an incredible week of golf.

Here are four takeaways from the appetizer round at iconic Pebble Beach:

USGA gives in ... so far

Whenever Phil Mickelson praises a U.S. Open setup and says the USGA "did a heck of a job," you know the association held back a bit. There are 39 players under par after Round 1, compared to only four last year. Additionally, there were 17 eagles made at Pebble Beach on Thursday, shattering the old U.S. Open single-round record of 12.

But scoring conditions can, and likely will, change quickly, even with very little wind in the forecast for the rest of the weekend. Pin positions can get tougher, greens can become faster, and the course can be stretched to its limit.

The USGA doesn't want the winning score to come close to 10-under par, so expect Round 1 to be the easiest day by far.

Hot putters lead

Rose leads the tournament after the first round, and he also tops the field in strokes gained: putting. He gained a whopping 5.52 strokes with his flat stick after only hit nine greens in regulation.

Schauffele and Aaron Wise are in a tie for second at 5-under, and they also rank second and third in strokes gained: putting, respectively.

Typically, hot putting rounds are tough to maintain over 72 holes, which opens the door for chasers who are performing well from tee to green. So don't be crowning Rose the U.S. Open champion just yet.

Koepka's 3-peat well within reach

Brooks Koepka got out of the gates quickly, recording birdies on four of his first six holes to make it seem like he'd cruise toward a third straight U.S. Open title. However, Koepka didn't keep his blazing pace going for all 18 holes, settling at 2-under for the day.

Sitting only three shots behind Rose, there's plenty of time for Koepka to climb back to the top. He was six shots off the lead after Round 1 last year before firing a second-round 66.

Koepka is now getting the second-best odds to win the U.S. Open behind Rose.

McIlroy in perfect spot

Strong starts at major championships are crucial, especially for Rory McIlroy. He carded a 3-under 68 in Round 1 at Pebble Beach, and the last three times McIlroy opened a major shooting 68 or lower, he's won each tournament.

After dominating the field in Canada, McIlroy's confidence appears sky high. He's only three shots off the pace, and in prime position to win his first major since 2014.