The French Open title may not mean as much in golf as it does in tennis, but you never would have guessed that from Siem's reaction at Le Golf National near Paris on Sunday night.
Although he did not jump in the lake like Thomas Levet last year - a wise move given that Levet broke his leg - the 31-year-old was simply thrilled to bits.
"I'm still in a dream," he said after beating Italian Francesco Molinari by a single shot.
"I've destroyed so many tournaments with stupid mistakes, but I never believed in myself like I did this week.
"I always show emotions, but I am a lot calmer now and I thought about my little daughter at home. I think that was one of the keys."
Siem was up against a field that included two former world number ones, Lee Westwood and Martin Kaymer, plus Graeme McDowell, Justin Rose and Ian Poulter.
He entered the week 120th in the rankings, but now he has smashed his way into the top 100 for the first time in his career.
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"This is one of the biggest tournaments we have on the European Tour, so to win it means so much."
He earned nearly £420,000 and the added bonus was a place in the line-up for Royal Lytham.
It will be only his second appearance, but on his debut at St Andrews two years ago he came 27th.
The bonus for Spanish Open champion Molinari in coming second - for the second time in three years - was that it moved him into a qualifying spot for the Ryder Cup.
He was part of the winning side at Celtic Manor and captain Jose Maria Olazabal would have been delighted to see him produce a sparkling closing 64 - the low round of the week - two months after winning in Seville with a last day 65.
English trio Poulter, David Howell and David Lynn tied for third, Poulter bogeying the final two holes after having a share of the lead earlier in the day.
Westwood was only 40th, but a closing 70 was good news after his injury scare on Saturday when he slipped on his way to the first tee.
The world number three had hurt his right knee and groin, but said before heading home: "The leg is fine - it wasn't a problem today [Sunday]."