TOKYO: US Open champion Juan Martin del Potro fell to a shock 6-4, 6-4 defeat by world number 189 Edouard Roger-Vasselin in the first round of the Japan Open on Tuesday.
The top seed refused to blame a stomach upset and jet-lag after losing his first match since the towering Argentine won his maiden Grand Slam title in New York last month.
"I didn't feel 100 percent," del Potro told reporters.
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"Everyone could see that. I had a little bit of jet-lag and a stomach ache but that's not the reason I lost."
"The other guy played good tennis and took his opportunities," added the world number five, runner-up in last year's Tokyo final. "Everyone can lose. It happens."
Del Potro blasted a forehand wide to hand Roger-Vasselin the first set and another into the net to gift the French qualifier a break in the sixth game of the second.
He called for the trainer at the change of ends and looked clearly uncomfortable when he trudged back out to receive in the next game which Roger-Vasselin held to love.
Del Potro's five-set victory over Roger Federer in the U.S. Open final ended the world number one's 40-match winning streak in New York.
The Argentine was a pale shadow of the player who stunned Federer, however, and his ordeal was quickly over when he slammed another forehand out on the first match point he faced.
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"It's unbelievable," said Roger-Vasselin after the biggest victory of his career.
"I know him better than he knows me. That's maybe how I won. I saw it was not a good day and thought 'why not today?' It was a great match for me."
Del Potro headed the field at the $1.25 million Tokyo tournament after Federer and world number three Andy Murray pulled out.
Earlier, Australian former world number one Lleyton Hewitt advanced to the last 16, the eighth seed beating Japan's Tatsuma Ito 6-1 7-5.
"I had to dig deep to get out of that second set," said Hewitt, who won the Japan Open in 2001 after the first of his two career Grand Slam victories at the U.S. Open that year.
"My focus now is on the Australian Open in January and getting myself in as good a nick as possible for that. The grand slams are still what drive me."