The 41-year-old German only got into the event last Sunday by winning a Challenge Tour event in France and has borrowed Richie Ramsay’s caddie for the big occasion.
Five shots off the halfway lead after opening with two 67s, Siem reeled off nine straight birdies in the third circuit before starting for home with a birdie-3.
Disaster then struck as he sent one sailing over the fence to run up a triple-bogey 8 at the 14th, but the four-time European Tour winner is made of stern stuff.
He punched the air in delight after bouncing back with birdies at the 16th and 17th, signing for a 70 and a six-under total to sit in the top 10.
“I think that was the best golf I've ever played in such an important tournament,” said Siem, who tied for 27th on his debut in this event in 2010 but had missed the cut on three subsequent appearances.
He reckoned that he’d hit every fairway from the start on Thursday until paying a heavy price for the first one he missed.
“I just tried to squeeze a 2-iron down there and didn't even think about out of bounds,” he admitted in recalling what had happened at the 14th.
“Tried to aim left of the bridge, hit a low chaser down there, hit it on the heel, and it cut it so much I couldn't believe it. It's very frustrating that that happened.
“I didn't want to do the same mistake with the second ball, so I hit it in the rough left; hit it to a perfect number from there.
“I wanted to have a full lob wedge into that flag and my ball was middle of the fairway in a divot. That's one of the toughest shots you can have with a lob wedge in your hand.
"That went in the bunker, semi plugged, downhill lie. No chance, but I'm very, very proud of myself that I was able to forget about it.”
Siem, who has Guy Tilston on his bag, lost his main tour card at the end of the 2018 season and considered packing it in as sponsors jumped ship.
Some players can’t get their head around being back on the Challenge Tour, but Siem isn’t one of them. Helped by that win in France, he’s on course to earn a place back at the top table next season.
Based on his endeavours so far this week, he might not be finished, either, in terms of wining titles on the main circuit. “I will give everything tomorrow to get a good result this week,” he said of his immediate task in hand.
Siem’s compatriot, Matthias Schmid, carded a 71 to sit in a tie for 44th on par, having posted the lowest round by an amateur in the game’s oldest event with his 65 on Friday.