Four-time champion David Ferrer has been knocked out of the Heineken Open, beaten in the semi-final in a huge upset by Yen-Hsun Lu of Taiwan.

The 30-year-old Lu knocked out the world No.3 6-4 7-6 to earn his first ATP Tour final spot, a day after making his first tour semi-final.

Ferrer made some uncharacteristic mistakes but Lu, ranked 62 in the world, was in top form and thoroughly deserved his victory.

He will play American John Isner, who struggled for half of his semi-final before overcoming Roberto Bautista Agut 3-6 7-6 6-4.

The third seeded Isner looked out of sorts right to the point where his opponent Roberto Bautista Agut served for the match in the second set.

But Isner unexpectedly found a way to break the Spaniard, easily won the subsequent tiebreak and dominated the third set to win 3-6 7-6 6-4 and make his second final in Auckland.

“The thing with me is that the body language can look pretty poor but I can still hold serve. It’s a luxury that I have with my serve,” he said.

“He was the better player for the majority of the match. I just stuck with it mentally, I guess. I still believed that I could get that match back to even.”

The victory now gives the American the chance to win his second title in Auckland, and a rare final appearance outside his homeland.

“I think this is only my third final outside the US, but I’ve always been very comfortable here in New Zealand.”

Isner is still being troubled by an ankle injury but said it was no worse than earlier in the week and it wasn’t affecting his game too much.

The doubles final will feature the top two seeded teams, both featuring an Austrian and a Brazilian, as top seeds Alexander Peya of Austria and Bruno Soares of Brazil take on Julian Knowle of Austria and Marcelo Melo of Brazil.