Orreal came from 4-0 down to win his deciding first round clash 5-4 against Phill Nixon, and showed all those battling qualities and more to go within a whisker of a place in the last eight. Orreal took the game's opening leg after Klaasen made a wayward start on doubles, although the young Dutchman won the nexy two legs to lead 2-1.
Orreal levelled in leg four, only for an 88 finish on double 12 and a double five checkout to see Klaasen move 4-2 up.
Further misses in the next from Klaasen allowed Orreal to pull back leg seven, before the former World Champion regained a two-leg cushion when the Australian wasted two darts at double top.
Orreal regained his range to level at five-all by taking the next two legs, and then moved 7-6 up by winning two of the next three.
Klaasen levelled on double four before taking out 125 on double top to edge back in front, and then take out 96 on double 18 - with Orreal sat on 50 - to move to a leg away from victory at 9-7.
Orreal took out a pressure 115 on double 19 to cut the deficit, and then finished 95 in the same bed to send the match into a decider.
The duo opened the final leg with 140s, but Klaasen had the edge with some strong scoring and landed double six to progress.
"It was harder than I wanted it to be but Pat played well," said Klaasen. "Every time I missed a double he checked out and he always came back when I got ahead.
"I had a good last leg and I'm glad that I won but it was not how I hoped it would be - I'm happy that I came through and know I have to be better in the quarter-finals."
Orreal said: "It's been a great ride. It's been the experience of a lifetime, fantastic and it's a shame it came to an end in this round.
"He hit his doubles when he needed to and he played very well. The crowd have been great to me and it was fantastic to play in.
"I'll go back to Australia and play in a few tournaments there, but I'm considering moving to England and playing for a few years in the PDC."