The second round of the 113th U.S. Open may not yet be complete, but the second day of this year’s Championship certainly did provide a complete dosage of fascination and intrigue. Once again, Merion was the winner, as this historic old course provided an extremely stern test. By the completion of First Round play on Friday morning, there was just five players under-par. (The fewest at that stage since 2007). By the suspension of Second Round play on Friday night, there were just two: Phil Mickelson and Billy Horschel.

Merion claimed many victims on Friday, with former champions Graeme McDowell and Jim Furyk succumbing to 36-hole totals of +13 and +16 respectively. It was a day for survival, as any round close to par would see a player rise abruptly up the congested leaderboard.

After the 1st Round was completed at lunchtime, in cold and damp conditions, the players finishing off from yesterday, who would be immediately thrust back into Second Round action, found the difficulty of the course increase as the wind picked up, with the breaking of the clouds. A beautiful and sunny setting remained in place for the remainder of Friday, but such beauty wasn’t replicated within the carnage witnessed on the course itself.

With such a troublesome canvas to perform on, the early afternoon starters found that even the most ordinary looking score would be beneficial to them. For Tiger Woods and Rory McIlroy, the world’s top two ranked players, even par rounds of 70 wouldn’t normally be considered anything greater than average. But this was not the case at Merion. They would match each other with two rounds of 73-70, for a three-over total, but it was one that would see them rise well into contention by Friday night.

Woods, who wasn’t at his best as he continued to struggle with a painful arm, reflected upon the examination that Merion provided in Round Two: "These pin locations are really tough. We knew the area they'd be around but we didn't think they'd be this severe."

Some would question the USGA’s setup, suggesting that it is tricked up, but nonetheless the high scoring has silenced those critics who believed that a soft Merion would be a birdie-fest, with scoring records potentially broken. As has become evident, a soft Merion is the only thing that has prevented this Championship from becoming a massacre, as complex and undulating greens make creating genuine birdie opportunities astonishingly difficult, despite often having just a wedge in hand.

But that wasn’t the case for Billy Horschel, winner in New Orleans in April, who provided a stellar exhibition of golf in the afternoon. The 26-year-old, in just his second appearance in his national championship, extraordinarily managed to hit all 18 greens in regulation in his second round. A quite stunning statistic, which led to a 67, that has him in a tie for the lead at 1-under.

Incredible play, but it was not something that Horschel was conscious of as even he struggled to remain fully comfortable on this testing layout: I was not in the zone, trust me," he said. "This golf course, even though it's soft, is still a tough golf course. I know what 'in the zone' is for me: I don't get nervous; I just see the shot and go. And I saw the shot and I went with it, but I was still nervous with a lot of them. Your misses here can be bad if you miss in the wrong spots. I was just focused on what I tried to do."

Someone at the different end of the career spectrum joined Horschel, who in 2004 shot a USGA Championship record of 60 in the U.S. Amateur, at the last minute on Friday. It was the five-time runner-up, and eventual First Round leader, Phil Mickelson.

Mickelson, who started the day at 3-under having shot 67 on Thursday, suffered a frustrating evening on Friday, as his superb iron play was let down by numerous missed putts. However, a stunning birdie on the 18th, which came moments after the horn was blown to announce the suspension of play at sunset, salvaged some positivity to take into the weekend. It saw him complete a 72, which gives him a 1-under 36-hole total of 139 that ties him with Billy Horschel.

Mickelson reflected on his day’s work: "I had many opportunities on holes I should be able to birdie but didn't capitalise, but I played the hard holes extremely well and made pars," said the man who will turn 43 on Sunday.

"I fought hard, hung in there, even though the score wasn't what I wanted. The birdie holes are a little tricked up and that doesn't give the guys playing well a chance to separate themselves from the field."

It was hard to separate from the rest of the field, and the leaderboard remains congested. Luke Donald, who like Mickelson had battled hard to shoot 72, Steve Stricker and Justin Rose are just a shot off the lead having completed 36-holes. Stricker, a 46-year-old, playing in just his seventh event of the year, will be attempting to become the oldest winner of the U.S. Open.

With the cutline yet to be officially defined, it is likely that the remaining field for the weekend will be separated by nine shots. A deficit that is not insurmountable, particularly on a course that will only continue to increase in difficulty with an excellent weather forecast on Saturday sure to provide firmer conditions.

Merion is a unique challenge in this modern game. It is not a power golf course, but one that demands complete control over the golf ball. Every shot must be controlled and hit with precision, otherwise you will be severely penalised. It will require immense shot making, which may favour someone like Justin Rose. But at least two-dozen players cannot be ruled out; not least Tiger Woods, Rory McIlroy and Ernie Els who are ominously just four shots back.

There are so many possible storylines. The most romantic would surely have to be a victory for Steve Stricker at the age of 46, or a birthday triumph for Phil Mickelson who can finally exorcise the memories of a record five-runner-ups this weekend.

Ultimately, it will all be determined by who can negotiate with Merion most effectively. It will require precision, patience, and a little bit of luck, and the protagonists will jostle throughout Saturday as they complete their second and then third rounds.

For some time now, Merion’s ability to stand the test of time was doubted by observers. After all, it has been 32 years since the U.S. Open was held at this tiny property. Although aided by an extremely penal setup, Merion has illustrated that it has stood that test.

As Jack Nicklaus, winner of 18-major championships, said recently: “Everybody says: ‘Merion’s going to be too short’. Merion’s not going to be too short."

He was right.