Del Potro, the 2009 US Open champion and Auckland victor, beat Ferrer 6-4, 6-4 to continue his strong form from the end of last season, which included victory in Stockholm and losing the Basel final to Roger Federer.
Waiting for Del Potro in the last four will be Spaniard Ferrer, who brushed aside ATP NextGen Finals champion Chung Hyeon of South Korea 6-3 6-2.
There were no break points in the first set but the Argentinian managed to break the Russian's first serve in the second set and felt it was an encouraging performance ahead of facing Ferrer.
A potential final against Juan Martin del Potro awaits.
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It was the first time such a play had happened for a score in postseason history . "No one wants to get run on", Flowers said. Mariota had some bad throws and plays, but he scrambled for some big first downs and made some big throws to win the game.
Having seen the 21-year-old Chung chase down everything Isner fired across the net in the second round, Ferrer was in no mood for long rallies and his plan to look for early winners paid off as he wrapped up the match in 74 minutes.
On the other side of the draw, Ferrer's compatriot Roberto Bautista Agut appears a safe bet for the final, drawn for a semi-final against Robin Haase.
In preparation for the Australian Open, starting January 15, Del Potro proved just too strong for the Canadian and won 6-2, 6-4.
Khachanov beat sixth seed Pablo Cuevas of Uruguay 6-2, 7-6 (4) to advance to the quarter-finals.
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Top-seeded and defending champion Jack Sock bowed out of the ATP Tour's ASB Classic in its second round Wednesday, beaten 6-3, 6-3 by unseeded German Peter Gojowczyk on a bad day for American players.
"I tried to not make mistakes, play in the right places, try and play as hard as possible with all the pressure", Bautista Agut said of the third-set tiebreak.
"Unfortunately the match can not have two winners and I'm so happy I won", said the relieved Spaniard, who sank to his knees at the end.
"If he has a good day, he is very risky as a lefty, he serves well and volleys".
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The unemployment rate at the end of past year amounted to more than 15 percent, the annual inflation rate was 6.4 percent. This has caused prices and taxes to rise across a range of essential goods and services, like fuel, cars and internet use.