Cilic reached the quarter-finals for the first time after coming through a marathon fourth-round match which lasted four hours, 38 minutes.

The 21-year-old fired 71 winners as well as 20 aces in the victory.

Cilic will now meet either seventh seed Andy Roddick or 11th seed Fernando Gonzalez in the last eight.

"Marin is playing great tennis and I think he did better than me," said Del Potro, who stunned Roger Federer to win last year's US Open in New York. "The serve in the end was my downfall.

"After four hours you have to really focus and he broke me early in the fifth. I had my chance but... he played very good. "

Cilic converted four of his 16 break-point chances and sealed victory on his first match point when del Potro's backhand went long.

"Today was my best so far," said Cilic. "Physically I think he was tired and he struggled, but I was feeling really good ... and I had some crazy fans."