The Malaysian qualifier overcame the fifth-seeded Alison Waters, a player more than good enough to influence the destination of this year's title, by 11-8, 11-5, 11-4 - a result as startling for its manner as its outcome.
Arnold trailed 3-6 in the first game yet outplayed Waters almost totally thereafter.
She relentlessly took advantage of the many strange errors made by the Englishwoman, who clinched the world team title for her country in December and has risen to a career-high world number three.
She departed the arena in Hull, northern England, quickly without speaking to reporters.
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Meanwhile a level-headed Arnold said afterwards: "I just wanted to go on there and see what I could do, without pressure. But I was definitely helped by what happened at the Asian Championships. That gave me confidence.
"I am quite surprised by what has happened though. I had played the qualifying, but it was Alison's first match, and it was before her home crowd, so obviously there was a lot of pressure on her.
That may be because she next plays Annie Au, the 11th-seeded Hong Kong player, to whom she lost in a close five-game semi-final in the Asian Championships.