The Englishman, the world number eight and highest ranked player left in the field at the semifinal phase, staged a grand rally after being outclassed by the wiry Thai world number 61 in the first three frames to earn a meeting with world number 22 Michael White in the final which is scheduled later today.
White ousted two-time world champion Mark Williams, the world number 15, 4-2 in an error-strewn first semifinal of the pound sterling 300,000 world ranking event.
Thepchasiya, who looked in splendid touch in the first three frames in which he compiled three half century breaks, was snookered out as he conceded 26 points through six fouls when trailing 41-47 in the seventh frame before Walden sealed his entry in the final with a decisive break of 23.
Walden, who had beaten Thepchaiya 4-1 in the Welsh Open second round last month, completed a 0-78 (78), 53-68 (62), 0-72 (68), 81 (81)-0, 73-36, 78 (78)-0, 91-47 win to enter the final against White.
White, thus, continued his dominance over idol Williams who played poorly for the most part and muffed easy pots to notch up victory after a poor start.