However, the 28-year-old Austrian surprised even himself as he stayed remarkably calm at Valhalla Golf Club and outscored his illustrious playing partner by two shots in the third round to charge into contention for the year's final major.
Wiesberger racked up six birdies in ideal scoring conditions on a wet and receptive layout to match the lowest score of the week, his six-under-par 65 lifting him into second place, one shot behind tournament leader Rory McIlroy.
"I know what I'm capable of doing," Wiesberger told reporters after finishing with a spectacular flourish, birdies on his last three holes that left him at 12-under 201.
"I know if I drive the ball well and don't get ahead of myself, I can play good golf.
"You know, I was kind of surprised that I was really calm today out on the first tee box. We had great crowds there for Phil. But hitting the first tee shot down the center obviously was a bit of a relief there."
What made Wiesberger's performance even more impressive was the fact that he had previously struggled under the intense pressure of major championships, missing the cut in four of his previous five starts.
"I've never played well in the majors," said the two-time European Tour winner. "I've played well in the other bigger events in Europe ... it's not the same, but you kind of get a feeling for what you have to do, how you handle yourself.
"It's just on a different level. But, you know, things are still the same. You're still out there with your caddie trying to do the best you can."
Wiesberger, who missed the cut at the U.S. Open in June and also at last month's British Open, felt he had probably benefited from being overshadowed by five-times major champion Mickelson as a fan drawcard.
"Maybe I stayed a bit in the background and let him do his thing," the smiling Austrian said of Mickelson, who is popularly known as 'Lefty'. "The crowd cheered for him. It was nice being in the situation. He's a great player, to feed off his performance as well.
"He was really nice with me out there today. We had a good couple of chats and a couple of laughs out there, so it was never really an uptight situation. It was really relaxed all the way round through."
Wiesberger has been paired with McIlroy for Sunday's final round at Valhalla and, given that they are friends, hopes he can replicate the relaxed approach to his game which he achieved in the company of Mickelson on Saturday.
"I've not been in contention in a major championship, so I don't know how it's going to turn out," said the Austrian. "I'm just trying to enjoy as I did today. I was in a great situation going into the round this afternoon and I felt quite calm.
"I'm friends with Rory and I'm sure it's going to be a nice situation out there tomorrow again. Hopefully I'll do the things well that I did today. Yeah, just enjoy myself. From now on, it's just a bonus, really."