The second-seeded del Potro will aim for his first Classic title since 2009, while the 29-year-old Bautista Agut will seek his second crown in three years.
Del Potro, who had not dropped a set all week, was unable to hold serve once in the first set as the Spaniard fought his way to a 6-1, 4-6, 7-5 win.
An wonderful forehand down the line victor moved Bautista Agut a point away from the crown and he sealed the deal when del Potro sent a forehand long, celebrating his second title in Auckland and the 7th in a career.
However, Del Potro broke at the ideal time - in the 10th game - in the second set to level the match.
Bautista Agut broke Haase in the seventh game of the second set but Haase immediately broke back and the set again went to a tiebreak.
"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.
Mate Pavic left and Oliver Marach celebrate
"I like to play here, I feel good being here in Auckland".
Juan Martin raised his game after a bad start, facing just 2 break points in the rest of the match, and he even had a chance to send the match into a deciding tie break, wasting a break point in the last game of the match that could increase his odds of winning the first Auckland title in 9 years.
Yet the Argentinian second seed recovered to take out the second set and repeatedly come close in the third, only to be broken late and lose 7-5.
"It's awesome to me to be playing tennis again after my wrist problems", Del Potro added. I knew it was going to be important, the beginning of the match. "We played a great match".
"And then I think my game improved a little bit. I hope I can play even better than tonight", del Potro said.