Unseeded American tennis player Lauren Davis is the first player through to the ASB Classic final after the third-set retirement of Jelena Ostapenko.

Ahead 4-1 in the third set, Davis was confirmed the semi-final victor when a flu-ridden and doomed Ostapenko threw in the towel.

The pair produced an error-strewn performance in Auckland, frequently failing to hold serve and producing a combined 16 double faults.

But Davis clearly got on top of her fatigued foe in the second half of the match, securing the second set before marching towards victory in the third.

The scores sat at 4-6, 6-4, 4-1 before Ostapenko's decision to retire.

Davis, ranked No.61 in the world, will now face either 2016 runner-up Julia Goerges or eighth-seed Ana Konjuh in Saturday's final.

"It feels amazing, it would be an incredible thing to be able to win this tournament," the 23-year-old Davis said.

Full of energy at the start despite her flu, Ostapenko blitzed her American opponent early, breaking twice before having her own service broken to leave the set poised at 3-2.

Ostapenko broke Davis for a third consecutive game to push further ahead, before having the chance to serve for the set at 5-2.

The Latvian couldn't take advantage at first, double-faulting as she was easily broken, but eventually held serve to snare the first set 6-4.

Davis, having been treated for a thigh complaint in the first set, nevertheless came out firing in the second, breaking Ostapenko to go 4-1 ahead.

A series of breaks and counter-breaks then saw the 23-year-old American secure the second set 6-4 and push the game into a decider.

In the third, Davis broke Ostapenko early to go 3-0 up before the 19-year-old called it a day at 4-1, disconsolate as she departed the court.