OAKMONT, Pa. - A squirrel ran onto the ninth green at Oakmont, looked around in confusion at the 10 U.S. Open golfers practicing their putting and tried to escape. He ran this way, then another, but still couldn’t find a way out or a tree to climb.

By the midpoint of Thursday’s opening round, more than a few in the U.S. Open field may feel the same way. Almost 5,000 trees are gone, but the toughness remains, with no place to hide from the troubles that await on every hole and, specifically, every green.

That was evident Wednesday during an 18-hole walk of Oakmont, where the only person who looked relaxed was a fan who found a shaded area near the No. 17 fairway, took off his shoes, used them for a pillow and took a nap. About 30 feet away, Phil Mickelson hit an approach shot during a practice round limited to 10 holes because of his sore left wrist.

No. 1, 482 yards, par 4: Tiger Woods calls this the toughest opening hole in American golf, but he didn’t play it Wednesday, working only at the driving range and practice green. Maybe he feels ready enough — or maybe it was the tobacco smoke. While many sports arenas, stadiums and ballparks, including Pittsburgh’s PNC Park, ban smoking, it’s allowed at an outdoor venue such as Oakmont. Five smokers were taking advantage by puffing away at a concession stand alongside the No. 1 fairway, irritating others waiting in line for a drink.

No. 2, 341 yards, par 4: At the 1994 Open, fans waited up to 20 minutes to traverse the lone spectator bridge that crosses the Pennsylvania Turnpike and connects holes 2-7 with the rest of the course. A second bridge was built for this Open, but only the golfers, caddies and tournament personnel can use it. Fans moving from No. 1 to No. 2 must wait to cross the No. 9 fairway to reach the spectator bridge then, on the other side, encounter a second delay getting to No. 2 as they wait for traffic to clear from the new bridge. The result? The same wait as before in some cases.

No. 3, 428 yards, par 4: The first hole where the half-acre Church Pews bunker comes into play, though it is more of a hazard for golfers who hook their drives on No. 4. Woods declined to practice hitting out of the bunker, saying, “I don’t really think that you should be practicing negativity.” On the green, longtime pro Bob Murphy, now an NBC announcer, took numerous practice putts alongside Brett Quigley.

No. 4, 609 yards, par 5: The first of two 600-yard-plus holes. As Quigley walks the fairway, one of the estimated 25,000 spectators asks why so few are practicing, making for little activity for long stretches at many holes. He replied, “A lot of guys are just playing nine today.”

No. 5, 382 yards, par 4: With so little shade, fans are crowding to sit under a covered picnic table not far from a large concession stand. There’s no other such respite on the course.

No. 6, 194 yards, par 3: There’s never been a hole in one in competition here.

No. 7, 479 yards, par 4: The field can only hope a late-afternoon thunderstorm Wednesday softens up the green on a hole that has been lengthened by 51 yards, creating a blind and very difficult second shot.

No. 8, 252/288 yards, par 3: Yes, it’s a par 3 — and it may play longer for the final round Sunday, according to the USGA’s Jim Hyler. One of the players told Hyler, “You could have a long drive and a closest-to-the-hole contest on the same hole.”

No. 9, 477 yards, par 4: Might be the only green in U.S. Open history where 30 golfers putt at the same time. Oakmont is unique in that the huge No. 9 green doubles as the practice green. USGA rules state there can be no practicing on a tournament green during competition. To police that, the USGA put down a blue line to differentiate between playing and practice areas. The penalty for violation: disqualification.

No. 10, 435/462 yards, par 4: As Mickelson begins his practice round, he is apologetic in declining autograph requests, blaming his inflamed left wrist. “I’m sorry,” he said. “Another week or so.”

No. 11, 379 yards, par 4: A blind, uphill tee shot to a tiny green. No wonder Mickelson spent so much time practicing there.

No. 12, 632/667 yards, par 5: The hole is so long, it runs nearly from the clubhouse to the Pennsylvania Turnpike — long enough to charge a toll. “It’s the epitome of a par 5,” said Masters champion Zach Johnson. In the 2003 U.S. Amateur, Trip Kuehne — who is in the U.S. Open field — reached in two with a 6-iron.

No. 13, 183 yards, par 3: Four deep bunkers guard a green with a serious tilt. Get above the hole and ...

No. 14, 358 yards, par 4: Johnny Miller missed a 10-foot birdie putt here on his final round in 1973, or else that famous 63 would have been a more famous 62.

No. 15: 500 yards, par 4: The Oakmont Country Club members tent runs alongside the fairway. As they watched the practice rounds, some members said the course’s setup isn’t nearly as difficult as it is for some member events. Among those members: Arnold Palmer. There’s another Church Pews bunker here, too.

No. 16, 231 yards, par 3: Probably the favorite hole of Richard Lee. He’s 16, just like the hole, the youngest player in the 156-man field and second-youngest ever in an Open. Not that it’s bothering him. “I’m not nervous at all,” he said.

No. 17, 313 yards, par 4: Woods still doesn’t know how it will play because of high rough on the left and the tilted green. “It’s a hole that going to be probably pivotal and very key to who wins the championship,” he said. By then, perhaps the sleeping man alongside the fairway will wake up.

No. 18, 484 yards, par 4: Palmer’s long walk up No. 18 in 1994, during his last U.S. Open round, is still remembered for the thunderous ovation he received. “The tough part was walking up 18,” Palmer said. “It was overwhelming.”

No. 19: Woods calls this his favorite hole. It must be — he spent considerable time signing autographs after a 45-minute stop on the practice green, something he doesn’t always do.