Del Potro won the first set 6-4, lost the second in a tiebreak before triumphing in the third via a tiebreak.
"I'm excited to keep surprising myself, the tour and seeing what I can do".
Federer said the defeat would hurt, but he took plenty of positives from the tournament as his record slipped to 17-1 this year.
He saved a third match point, softly stroking a passing shot down the line, before a crushing forehand victor had the usually elegant Federer scrambling desperately across his baseline. I'm No6 in the world, I think?
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"Well played today, tough match but well done you deserve it, you were the better player in the end", he said.
Del Potro has regained form over the past two seasons but although he entered Sunday's final on a 10-match winning streak, he was still inevitably playing underdog to Swiss world No1 Federer. "I played a great match and had the chance the win in two sets but I made a mistake with my forehand which I couldn't believe that mistake", Del Potro told Sky Sports after recording his seventh win over Federer in 25 career meetings.
The Swiss ace was favorite to win the title going into the summit clash, but failed to convert three match points as the Argentine fought back to end Federer's unbeaten start to the season.
Del Potro raced out of the gate with an aggressive, in-your-face game, gaining the first set lead.
"It's so big", Del Potro said. It really could have gone either way, until it became obvious in the third set tie-break that it would go del Potro's.
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Salah is a winger, and he's played less, and he's still scored 4 more than the diving moron that is Kane. Honestly, both are fantastic, and now Salah is in a very good moment, one of the best in Europe.
Del Potro complained to the umpire at the changeover about the noise from a crowd that was largely supporting Federer in an increasingly feisty encounter.
With Federer 40-15 up on serve and leading 5-4 having broken Del Potro in the previous game, it was again the Argentine's huge forehand which began dictating the points, leveling the score at deuce.
When asked what he expected of himself for the remainder of 2018, he said: "I don't know, I'm just going my way and I'm very pleased with my level of tennis".
"Roger and I were nervous during the whole match, and we felt that on court. It's hard to describe with words, it's like a dream", del Potro said on court after extending his own winning streak to 11 matches.
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A longtime assistant in the league, Drew has also had head coaching stints with the Atlanta Hawks and the Milwaukee Bucks. Lue cited dealing with "chest pains and other troubling symptoms, compounded by a loss of sleep, thoughout the year.