Venue: Melbourne Park Date: 18-31 January
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Del Potro defeated Cilic at the US and Australian Opens last year
US Open champion Juan Martin del Potro has been knocked out of the Australian Open after losing 5-7 6-4 7-5 5-7 6-3 to Croat Marin Cilic in Melbourne.
Cilic will face Andy Roddick after coming through a marathon fourth-round match after four hours and 38 minutes.
The 21-year-old fired 71 winners and 20 aces in his victory over the Argentine.
Seventh seed Roddick weathered a storm against Chile's Fernando Gonzalez before winning 6-3 3-6 4-6 7-5 6-2 in a big-hitting contest under the lights.
The three-and-a-half hour contest ended when a disheartened Gonzalez, who lost the fourth set on a HawkEye challenge, double-faulted at just gone 1am local time.
Roddick will face Croatia's Cilic as he bids to reach the semi-finals for the fourth time.
"I got a little lucky tonight - and sometimes it's better to be lucky than good," admitted the American.
I think he was tired and he struggled, but I was feeling really good ... and I had some crazy fans
Fourth seed Del Potro's luck seemed to finally run out over on Hisense Arena.
The 21-year-old Argentine came into the tournament with a wrist problem, and dropped sets in each of his opening three matches - including a gruelling five-set win over James Blake.
The effort seemed to catch up with him against Cilic, and he received lengthy treatment on his left foot during the contest.
However, he paid tribute to his opponent.
"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.
"I will go home and see my doctors there and have some rest and recovery before the next tournament."
Cilic dropped the first set but took a 2-1 lead as Del Potro struggled to find his top form.
Del Potro fought his way back in the fourth set, breaking Cilic in the 11th game to love then managing to hold serve to send it into a decider.
Cilic stormed into a 5-2 lead in the final set and although Del Potro then held his serve, the end was not long in coming, the Croat sealing victory on his first match point when his opponent fired a backhand 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."
Gonzalez was cheered on by his own band of raucous fans in the day's final match on Rod Laver Arena, and they were sensing a minor upset when the Chilean hit a hot streak to take a two sets to one lead.
The temperamental Gonzalez took his frustration out on his racquet
But Roddick showed admirable resolve to stay with his opponent in the fourth set, saving a break point early on with a huge serve out wide.
The American saw four set points come and go at 5-4, but refused to concede defeat.
He battled back from 40-0 down at 6-5, and got his reward when, on a fifth set point, his forehand was called wide but subsequently overruled by HawkEye.
The umpire gave the point - and with it the set - to Roddick, correctly ruling, much to Gonzalez's disgust, that the Chilean had let the ball go before the out call came.
The temperamental Gonzalez, a finalist in Melbourne in 2007, took all his frustrations out on his racquet when he dropped serve early in the fifth set, and he could not recover, the 11th seed racking up 49 unforced errors in the match compared to Roddick's 17.