The Australian sealed the 10-6 victory in Llandudno as he looks to make a major statement ahead of the upcoming world championships.
Robertson looked in the form of his life as he hit three centuries on his way to taking a 7-0 lead.
However, just as the Australian looked nailed on for the semi-final, Allen dug deep and began to mount a spectacular comeback.
After scrapping out his first four frames, the 36-year-old then won two of the next three to make it 8-6.
Allen's comeback was reminiscent of the previous night's encounter which saw John Higgins bounce back from 8-4 down to beat UK champion Zhao Xintong.
However, on this occasion, the comeback didn't fully transpire as Robertson held his nerve and rose to the challenge in spectacular fashion. Back-to-back scores of 121 and 130 - the latter being the highest break of the tournament - sealed a memorable 10-6 victory for the former world number one.
Speaking to World Snooker after his thrilling win, Robertson said: "I'm very proud because Mark asked many questions of me tonight. I had many answers, but he had answers to those! His safety was incredible and shut me out.
"The key is to be patient and to not lose your cool. I didn't make any disastrous mistakes, I missed a couple of tricky balls and that was it. I am experienced enough to know things can turn around, but the most important thing is to be ready when that mistake comes.
"This evening provides more information than if I coasted through 10-1 tonight. The fact I came up with the goods when it really mattered sends a lot of positive signals to me, knowing I can withstand anything. I can put my foot down and run away and I can respond really strongly when someone comes back as well."
Next up for the 40-year-old is another possible blockbuster against either Mark Williams or Ronnie O'Sullivan.