Juan Martin del Potro is getting the Fed out…

The 28-year-old Argentinian tennis player and 2009 US Open champion defeated Roger Federer in four sets, 7-5, 3-6, 7-6 (8), 6-4, at Flushing Meadows on Wednesday night.

del Potro’s upset victory prevented a much-hyped about first U.S. Open matchup between Federer and Rafael Nadal.

Federer entered the quarterfinals with an 18-0 Grand Slam record this season, including titles at the Australian Open and Wimbledon to raise his record count to 19 major championships.

But he injured his back at a tournament last month, curtailing his preparation for the U.S. Open, and he wasn’t quite at his best for stretches. He needed five sets to win each of his first two matches – and Wednesday, he succumbed to the same formula of massive forehands and booming serves that del Potro used when he upset Federer in the final eight years ago.

Arthur Ashe Stadium was packed, and both men had loud groups of supporters. Federer’s fans would cheer for del Potro’s faults, considered bad etiquette in tennis. del Potro’s faction would break into raucous, soccer-style songs of “Ole, ole, ole, ole! Del-po! Del-po!”

“Well, I think it’s my home court, too,” del Potro said in an on-court interview.

The 2-hour, 51-minute contest was filled with some sublime shot-making by each player, and some real moments of shakiness for Federer, whose forehand in particular was problematic. The turning point was the third-set tiebreaker, which Federer was a single point from winning on four occasions.

At 6-4, del Potro hammered a good return that caught Federer off-guard, resulting in a forehand into the net. At 6-5, del Potro delivered a service winner. At 7-6 – set up by a double-fault from del Potro – Federer missed a backhand, and his wife, Mirka, put her hands to her temples, before standing to offer encouragement. At 8-7, Federer’s fourth and last set point, del Potro hit a huge forehand winner.

That began a run of three points in a row for del Potro to claim that set, the last when Federer pushed a backhand volley long.

The suspense in the fourth set was brief: At 2-all, Federer dumped an overhead into the bottom of the net to gift a third break point of the game, which del Potro converted with a stinging cross-court backhand return winner to nose ahead for good.

Del Potro showed no ill effects from his 3 1/2-hour, five-set comeback victory in the fourth round – or the illness that’s been bothering him and had the 1.98-metre Argentine coughing into a towel late in the second set. His forehands were powerful and precise, including one reflex pass hit so hard and so close to Federer’s head that he ducked out of the way.

Federer was uncharacteristically off at moments, including a very bad forehand volley that was way off the mark and set up del Potro’s match point.