Second-seeded del Potro, champion in 2009, beat four-time champion and eight-time semifinalist David Ferrer 6-4, 6-4 in just under two hours Friday while Bautista Agut, the 2016 champion, defeated Robin Haase 6-7 (7), 7-6 (3), 7-6 (5) in a three-hour marathon.
"Today was a very tough match, I don't know how my body will be tomorrow, but now I want to enjoy the win and we'll see", Bautista Agut said.More news: Thomas Fire, California's Largest On Record, Is Now 100 Percent Contained
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.
Del Potro said he found it hard to play in humid conditions in the daytime Saturday after playing his previous matches at night.More news: Serena Williams saved her own life during six days of postpartum hell
The second-seeded del Potro - a red-hot favourite for Saturday's Auckland final - fell 6-1 4-6 7-5 to his Spanish opponent, failing to fire early. But del Potro took advantage of the only break point of the second set to level the match.
Despite his loss, Argentinian Del Potro - who was eyeing a 21st ATP crown - is set to return to the top 10 in the rankings.More news: Warm weather could cause ice jams
"I'm exhausted", Del Potro said in a court-side interview. "It's a good start for me", he said. "Juan Martin is a great champion, he's very tough to beat, and I played two awesome games in the last two games of the match". I think I played very aggressive... Juan Martin netted a backhand to get broken in game number 2 but he fired up his mighty forehand to get back on the track in the following game, breaking back with a volley victor and reducing the deficit to 2-1. Bautista broke again in the sixth game to lead 5-1 and comfortably served out the set in a game including his seventh ace. He again saved a break point before winning the set in 50 minutes.