Caroline Wozniacki was "exhausted" after beating American duo Sofia Kenin and Sachia Vickery on Saturday to set up an Auckland Open final against Julia Goerges.
Two days of rain put the quarter-finals back to the weekend and Wozniacki was able to pass the test of stamina, getting past teenage wildcard Kenin 4-6 6-2 6-4 to reach the last four after a short weather delay in the final set.
The top seed, full of confidence after ending last season with her biggest tournament victory at the WTA Finals, then ended qualifier Vickery's run to move into the title decider.
Vickery, the world number 122, shocked fourth seed Agnieszka Radwanska 6-2 6-2 in the quarter-finals, but went down 6-4 6-4 to Denmark's Wozniacki.
"I'm exhausted, if I'm being honest," said world number three Wozniacki. "It's a tough start to the season - you kind of feel it after the first match...in the back, and the legs, and then we didn't play for a couple of days, and I said, 'Oh, it's nice.'
"Then coming out here and playing three sets this morning, and then two sets when we ran a lot - I'm not 17 anymore, I can feel it for sure!"
Second seed Goerges saw off Polona Hercog 6-4 6-4 before also sealing a straight-sets victory over Su-Wei Hsieh, winning 6-1 6-4.
The in-form German will be going for a third consecutive title on Sunday, having won in Moscow and Zhuhai on her previous two outings last October.