Slovakia’s Daniela Hantuchova has caused the first major upset at the ASB Classic tennis tournament, ousting No.2 seed Sara Errani in straight sets.

World No.62 Hantuchova rattled off the last six games to ease past her 14th ranked Italian opponent 6-3 6-2.

Errani, who is returning from a rib injury that disrupted her pre-season preparations, fought back from losing the opening set to get an early break in the second.

But Hantuchova then took over the contest between two former world No.5s to win in one hour and 15 minutes.

She will face either another Italian, Francesca Schiavone, or Polish qualifier Urszula Radwanska in the second round.

Earlier on Monday, sixth seed Sloane Stephens made it three wins out of three for Americans on the opening day.

Stephens, the world No.36, took just over an hour to get past Spain’s Silvia Soler-Espinosa 6-3 6-1.

Her next opponent will be fellow American Lauren Davis, who overcame compatriot Shelby Rogers in a lengthy three-setter.

While Stephens’ win was relatively straight-forward, Davis had to mount a big comeback before prevailing 1-6 6-4 6-3.

There are seven Americans, headed by seven-time grand slam winner Venus Williams, in the 32-strong main draw, making them the largest national contingent.

Also among them is wild card Taylor Townsend, who downed former champion Yanina Wickmayer in three sets.

Townsend, 18, produced some heavy shot-making, none more so than in the second game of the final set as she broke her Belgian opponent to love.

The world No.104 went on to close out a 7-5 3-6 6-0 win over Wickmayer, a player ranked 30 places above her.

“I was really happy I was able to fight through,” the former world No.1 junior said.

“I was fighting through a lot of different things – nerves, myself, the old things, some of the new stuff I had been working on.”

Townsend now awaits the winner of the match between top-seeded Dane Caroline Wozniacki and Israeli qualifier Julia Glushko.