Haase won his quarter-final 6-4, 6-4 against Germany's Peter Gojowczyk who had beaten the top-seed and reigning champion Jack Sock the previous day.
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.
"It was an unbelievable match today", said Bautista Agut, who was presented with a ceremonial "taiaha", or Maori wooden staff. "Juan Martin is a great champion, he's very tough to beat and I played two wonderful games in the last two games of the match".
"I played well and I'm looking forward to doing well in Melbourne".
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"I tried to serve hard to hit hard with my forehand, to play some slices and drop shots, it's never easy against David because he runs a lot and he's a fighter".
"It's a good start for me", he said.
Juan Martin del Potro reached his 30th career ATP final as he wore down David Ferrer to prevail at the Auckland Open.
"It's just the beginning of the year, but I think I'm getting closer to my best the level again and just trying to focus from match to match and trying to play again my tennis like I played in the last years, especially 2016".
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Del Potro and Ferrer's rallies were generally shorter as they slugged it out from the baseline.
"I was a break up in every set and right after I got (it), he breaks back".
"Roberto was just playing so well", said Del Potro, who added he had played most of his matches in Auckland at night and was slow to adjust to the hot and humid conditions on Saturday.
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.
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