Defending champion Yanina Wickmayer beat China's Peng Shuai 3-6, 7-6 (3), 7-5 on Friday to advance to the ASB Classic final.
Wickmayer struggled with her serve, squandering her first match point with her ninth double fault before wrapping up the victory. She next plays Greta Arn, who beat Julia Goerges 7-6 (3), 6-3.
The second-seeded Wickmayer lost her cool with linesmen and the chair umpire after a series of disputed calls.
Arn, who beat top-seeded Maria Sharapova in the quarterfinals, reached her second WTA Tour final and first since she won the Estoril title in 2007.
The Hungarian only sneaked into the main draw at Auckland when three players immediately ahead of her withdrew and came to the tournament without her coach who told her it was too far away "unless I buy a Concorde."
"I'm living a dream — I'm definitely playing some of the best tennis of my life," the 31-year-old Arn said.
Arn has been on the professional circuit since 1997, barring the three-year break she took from 2002 to work in advertising. She said she had to take the time off to rebuild her finances after an agent left her "in red figures."
Until this week, she had won a meager $600,000 in 14 years on the professional circuit and her last big upset win, before Thursday's over Sharapova, had been over Mary Pierce nine years ago.
The ASB Classic has a total purse of $220,000, low by WTA standards.
Wickmayer broke Peng's serve in the fourth game of the marathon second set but Peng broke back immediately and had two break points on Wickmayer's next serve, which the Belgian saved with successive aces.
Peng broke again to lead 6-5 and to serve for the set but faltered for the first time, allowing Wickmayer to clinch the set in a tiebreaker.
Both players dropped serve at the start of the final set but Wickmayer broke again to lead 3-2. Peng broke back to tie the match at 5-5 as Wickmayer double-faulted at match point.
Wickmayer broke for a 6-5 lead and, after a double fault and argument with the chair umpire, recovered her composure to serve out the match.
"It was a really, really tough match," she said. "She had a lot of chances in the second set to close it out. I just had to keep my focus and I pulled it through."