The Hungarian faces fourth seeded German Julia Goerges, who last night downed Ukranian wildcard Kateryna Bondarenko 5-7 6-2 6-2.
Arn, 31, booked her place with the biggest upset the tournament will witness this week, ousting the top seed and the event's premier drawcard, Russian Maria Sharapova, 6-2 7-5.
It was a result that Arn rated as the biggest of her career, eclipsing her upset over another multiple Grand Slam winner, Mary Pierce, in 2002.
"I was just shaking," she said of serving for the victory.
"I didn't feel nervous during the match, only in the last game. I started to think, oh my god, I can beat Maria Sharapova."
Arn's passage into the last four follows a year in which she jumped more than 140 places in the world rankings to her present 88.
While Sharapova's superstar status made her the centre of attention in Auckland, Arn has largely slipped under the radar.
She arrived on her own, her coach deciding New Zealand was too far to come from Europe, although he maintains constant contact.
"He doesn't sleep, he's awake at night, watching live scores [on the internet]," Arn said.
"He calls me every single time about what I should do. It keeps me together."
Arn also had to survive several days without her luggage, but did get a break when late withdrawals meant she got direct entry into the main draw rather than having to qualify.
Not that qualifying holds any fears for her.
She made it into the main draw via that route eight times last year and she won her only WTA Title, in Portugal in 2007, as a qualifier.
Arn first broke into the top 100 in 2002, but then had to put her career on hold because of "an agent problem", when bad investments left her in debt.
She joined the media ranks, doing behind-the-scenes television work at tennis tournaments.
"It was fun and I was very happy that I did that," she said.
"I know how it is if I don't play tennis. I know the other side, normal life."
She returned to the circuit at 26 and remained outside the top 100 until her big move back up the rankings last year.
Of Hungarian-German parentage, Arn represented Germany until switching her allegiance three years ago.
The move led to competing in the 2008 Beijing Olympics, which she described as her biggest adventure.
