SIXTH seed Juan Martin del Potro stunned world No1 Roger Federer to win his first Grand Slam in an epic US Open final last night.<br /><br />The Argentinian 20-year-old twice came from a set down to end Federer’s five-year reign of glory at Flushing Meadows and deny him a 16th major title. <br /><br />The Swiss star, arguably the best player ever to grace the game, looked on course for yet another championship. But the 6ft 6in youngster refused to give in and, exhibiting the same brutal ground strokes that saw off Rafael Nadal in the semi-final, ground out a 3-6, 7-6 (7-5), 4-6, 7-6 (7-4), 6-2 victory.<br /><br />Federer took a 6-0 head-to-head advantage into the match and wasted no time in asserting his authority, breaking in Del Potro’s first service game and serving out to take the set without fuss.<br /><br />An easy triumph looked on the cards when Federer began the second set in a similar vein, breaking instantly to go 2-0 up. But at 5-4 the set turned on a moment of drama. Del Potro challenged a call and won a break point, which he took before winning the tie-break to level.<br /><br />An invigorated Del Potro then broke first in the third to go 4-3 up but Federer responded instantly with a double break, and the set, when his opponent double faulted.<br /><br />Federer was now in the ascendancy, forcing break points early in the fourth, but it was Del Potro who struck first to go 4-2 ahead. Federer again hit back straight away to level at 4-4, but Del Potro was too strong in the ensuing tie-break, and forced a deciding set.<br /><br />Momentum stayed with the youngster, who swept into a 3-0 lead. Federer stopped the rot, and at 5-2 down saved two set points but, crucially, could not defend the third, which handed Del Potro the prize.