The course previously served as host of the 1992 PGA Championship.
We have had four separate winners of the PLAYERS (Webb Simpson), Masters (Patrick Reed), U.S. Open (Brooks Koepka) and Open Championship (Francesco Molinari).
'I feel somewhat under the radar this year.
"I have the time to look around a little more closely", he said.
Marc Leishman is next best of the Australians at two under midway through his round, while former world No.1 Adam Scott is a shot back at one under.
He returns almost as much of a mystery as when the year started. I've done a lot of good things.
'But at the same time this tournament will always be circled until I'm able to hopefully win it some day.
"It's hard to remember a course I've played that has so many doglegs left", he said.
The two-time major champion will be looking to add a third major title this weekend. "It doesn't necessarily mean you're going to play well, but it certainly helps you to play well".
Thomas and Rose could overtake Johnson for world number one this week, but Rose would need a victory and Thomas no worse than a solo second finish. "I missed a couple of 7-8 footers for par which was frustrating. but you can't hole everything, and I did hole my fair share today".
Want to play a round in St. Louis?
Longtime golf instructor Peter Kostis thinks Tiger Woods has done "exactly what he needs to do" in remaking his swing after spinal fusion finally allowed him to overcome his back problems.
Meanwhile, European Ryder Cup captain Thomas Bjorn has pulled out on the eve of the tournament with a back injury.
Ten years after his last major victory and five years since his last triumph, Tiger Woods is back on the prowl and a new generation of rivals couldn't be more thrilled.
The USGA is often criticized for its U.S. Open setups, whether that's because of long, thick rough or penal green speeds that are created to produce a specific winning score.
"You can learn just as much from winning a major as I can from winning in Akron last week, or winning the Sony with a seven-shot lead on Sunday", Thomas said.
"In golf, you just have to be an eternal optimist", McIlroy said.
But he is yet to win a major - or a tournament in the United States. At the end of the day, a lot of careers are defined on major championships.
"I would rather do that than have one hot week and, yes, you win a major".
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