Jack Sock is through to the last four of the ASB Classic for the second successive year.

The American progressed via another three set battle, outlasting Frenchman Jeremy Chardy 5-7 6-4 6-3.

Like he did on Wednesday against Ryan Harrison, Sock showed considerable nerve.

Chardy took the first set and had a chance for a break late in the second, before Sock came storming back.

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The Frenchman fought hard - saving an incredible 21 break point opportunities across the match - but was eventually overwhelmed by the power and accuracy of the American, especially off the forehand wing.

"I've played matches with a bunch [of tiebreaks] but that was crazy," said Sock. "It was a credit to him but eventually I got some."

Sock forced 11 break point opportunities in the first set, but was unable to convert any of them, before Chardy took his only chance on the Sock serve to seal the set.

The second set threatened to follow a similar pattern.

Sock put regular pressure on the Chardy, but couldn't achieve the breakthrough until late in the set.

The third set was more of a procession, as Sock sprung to a 5-1 lead and eventually sealed the result in 129 minutes.

Sock will face compatriot Steve Johnson in Friday's semi final.