* Del Potro stunned by plucky Frenchman
* Chardy to meet Italy's Seppi (Writes through with quotes, adds byline)
By Ian Ransom
MELBOURNE, Jan 19 (Reuters) - Former U.S. Open champion Juan Martin del Potro became the highest seed dumped out of the Australian Open on Saturday when he was stunned 6-3 6-3 6-7 3-6 6-3 by France's Jeremy Chardy in their third-round clash.
The sixth-seeded Argentine, the only player outside of the 'Big Four' of men's tennis to have won a grand slam in the last eight years, had come into the match in ominous form, conceding only 13 games to his first two opponents, the lowest in the men's draw.
After 225 minutes on Hisense Arena, though, it was the unfancied Chardy, a 36th-ranked clay court specialist with a solitary ATP title, who was toasting one of the finest wins of his career.
"From the first point to the last point, I played exactly the same. I stayed calm, and very focused so that's why I won today," the 25-year-old beamed at reporters, after setting up a fourth-round clash with 21st seed Andreas Seppi of Italy.
"It's a big win for me, and maybe the best of my career ... I'm so happy."
The 6ft-6in (1.98 metres) Del Potro was dismantled by the simplest of tactics, lured inside the court by sliced backhands and then picked off by the Frenchman's forehand.
The Argentine "Tower of Tandil" served his way back into the match, but lost the decisive break at 4-3 in the fifth set, allowing Chardy to calmly serve out with an ace on match point.
Del Potro, wearing a bright blue and white polo shirt in the Argentine colours, was philosophical at his post-match media conference.
"In the end when he had the chance to break me, he did," he said. "I think that was the key. In the end, he played better in the important moments, and he took the chance to win the match."
Since upsetting Roger Federer in the 2009 final at Flushing Meadows, Del Potro has struggled to reach his peak after a long battle with a troublesome wrist injury.
He returned as a force in men's tennis last year, however, winning four titles and reaching the quarter-finals at Melbourne Park and the U.S. Open to finish the season in the top 10.
Even after shocking world number one Novak Djokovic to win the singles bronze at the London Olympics, Del Potro has been circumspect about his chances of adding a second grand slam title.
"I have to work hard because the season just started," Del Potro said. "I believe in my game, myself. I will work very, very hard for the future." (Editing by Patrick Johnston and Nick Mulvenney)