Top seed David Ferrer snuffed out a determined challenge from fellow Spaniard Guillermo Garcia-Lopez on Thursday to reach the semi-finals of the Auckland Open.
American John Isner, seeded third, also advanced into the final four, along with Taiwan's Lu Yen-hsun and Roberto Bautista Agut of Spain.
World number three Ferrer, who is seeking a record fifth title in the warm-up event for this month's Australian Open, looked to be cruising to victory as he took the first set 6-3, then broke Garcia-Lopez in the first game of the second.
But his countryman, ranked 58 in the world, held on stubbornly and snatched two service games off Ferrer late in the set to win it 7-5 and force a decider.
The pair exchanged breaks again in the third set but Ferrer, covering every inch of the court, was able to gain an advantage and close it out 6-4.
"He played unbelievable tennis. I'm very tired now," said the 31-year-old, who will meet Lu in the semis after the Taiwanese world number 62 defeated American Steve Johnson 6-4, 6-4.
Isner, a winner in Auckland in 2010, emerged victorious from a ferocious arm wrestle with fifth seed Phillipp Kohlschreiber of Germany which did not produce a single service break.
The pair matched each other blow-for-blow over two hours 25 minutes of play, Isner finally edging ahead to win 6-7 (4/7), 7-6 (7/4), 7-6 (7/5).
The lanky American, who was nursing a sore ankle late in the match, said he was fortunate to prevail in a contest that could have gone either way.
"I was up against a gutsy opponent so I knew I have to give it everything I had," he said.
"It wasn't necessarily pretty for either of us but when you're playing guys as good as him it comes down to a point here and there. I got a bit lucky today."
Isner's semi-final opponent is Bautista Agut, who beat American Jack Sock 6-1, 6-3 in a stop-start match hit by rain interruptions.
"It's not easy when you stop and get cold then start again," the Spaniard said.
"But I kept focus all the match and played good, that's what got me through."