The German is a four time European Tour winner but is currently plying his trade on the European Challenge Tour and claimed his maiden win on that circuit on Sunday to secure his place in the field at Royal St George's Golf Club.
His victory at Le Vaudreuil Golf Challenge ended a seven year winless drought and the 41-year-old carried that momentum into a three under par effort with five birdies and two bogeys.
And while he could have been forgiven for celebrating with veteran caddie Guy Tilston, Siem was determined to get some rest and go low again on day two.
"There's no time for that," Siem said when asked if he would be celebrating. "I'm going to have dinner. This week is too important. It's my 14th week in a row.
I was happy on the golf course and I turned things around. I'm just so grateful to be here now
"I'm tired, so I might have one beer with Guy later, but a lot of sleep is really important."
He added: "It's always nice to have a good start. It takes the pressure off a little bit off.
"I think you have to be patient on this golf course. They can set it up so tough with the flags and I'll just try to make as many pars as possible tomorrow and then we'll see what happens.
"I was happy on the golf course and I turned things around. I'm just so grateful to be here now.
"There's no reason to give up and I felt my iron play was good enough to turn things around and I'm really, really pleased to be three under par."
Jack Senior also secured his place in the field on Sunday at the abrdn Scottish Open and was bogey free as he teed it up in his fourth Open looking to make the cut for the first time.
"I just continued my form from last week really to be honest," he said. "I drove the golf ball brilliantly last week and I've just gained so much confidence with my driver.
"Obviously my iron play was pretty much on point as well but the putter was a little cold on the back nine, gave myself a couple of really good looks but hopefully the putter warms up tomorrow."