Roberto Bautista Agut has muscled his way into the ASB Classic final with a 6-7 (7-9) 7-6 (7-3) 7-6 (7-5) victory over Dutchman Robin Haase in Auckland.
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.
Both players will now turn their attention to next week's Australian Open in Melbourne, where Bautista Agut will play compatriot Fernando Verdasco in the first round.
Despite his loss, Argentinian Del Potro - who was eyeing a 21st ATP crown - is set to return to the top 10 in the rankings. After almost three hours on court, the classic encounter was settled by a Bautista Agut lob that landed on the line, ending 6-7 (7/9), 7-6 (7/3), 7-6 (7/5)."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.
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"Unfortunately the match can't have two winners", an exhausted Bautista Agut said after the two hour, 52 minute clash. "I didn't expect to reach a final in my first tournament", said del Potro, the 2009 US Open champion.
"When I had to play in the sun it was very tough for me", del Potro said.
"Juan Martin is very tough to beat and I think I just played two awesome games at the end of the match". I think the crowd enjoyed the game.
"I have many good things to celebrate - of course, when you lose in a final, it's not happiness, but the tennis life continues". 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. He again saved a break point before winning the set in 50 minutes.