"I didnt expect to reach a final in my first tournament of the year but I played well", Del Potro said.
Speaking on court afterwards, Del Potro, who won the title in Auckland in 2009, said: "I'm exhausted, I'm so glad to be in the final after 10 [nine] years. I played two awesome games at the [end] of the match".
Roberto Bautista Agut is not your typical Spaniard.
"Today the conditions were hard and he was a hard opponent, serving very well and a lefty, playing with a very flat backhand", Bautista Agut said. I've been fighting a lot on the court...
Ferrer is yet to drop a set in Auckland and has shown glimpses of his old form that saw him win the tournament in 2007, 2011, 2012 and 2013. So I'm really happy and proud to win the first title in 2018.
However, Bautista Agut romped through the first set in 31 minutes as he broke the Argentine three times and produced numerous forehand winners.
Neither player offered many opportunities in the third set until the 11th game, when the Spaniard converted the second of his break points to take a 6-5 lead and then held serve to triumph in a little more than two hours.
Del Potro fell to 20-10 in career finals, posting a 1-1 record in finals in 2017.
In a battle between the 2016 and 2015 winners, the more recent champion Roberto Bautista Agut proved too good for Jiri Vesely with a 7-6, 6-2 victory to enter the semi-finals where he will play Robin Haase of the Netherlands on Friday.
The 18-year-old Canadian is now 0-2 to start the year after dropping his opening match of 2018 on January 2 at the Brisbane International to Britain's Kyle Edmund in three sets.
After an entertaining final set, Bautista Agut broke for 6-5 and had to save a break point as he served out the match.