Del Potro lunges for a forehand during his first-round defeat
Top seed Juan Martin Del Potro suffered a shock first-round defeat at the Japan Open in Tokyo.
The Argentine, who recently won the US Open by beating Roger Federer, was beaten 6-4 6-4 by France's Edouard Roger-Vasselin, the world number 189.
"I didn't feel 100 per cent. Everybody could see that. I had a little bit of jet-lag and a stomach ache but that's not the reason I lost," Del Potro said.
"The other guy played good tennis and took his opportunities."
It was the 21-year-old's first match since overcoming Federer in five sets in New York to win his first Grand Slam title and he needed treatment from the trainer after his serve was broken in the sixth game of the second set.
But the defeat was an unexpected setback as he strives for the consistency to improve his world number five ranking still further.
"I felt so happy after the U.S. Open. We had a good celebration with family and friends in Argentina, but I am disappointed about today's result. I have to keep improving," he added.
Vasselin served nine aces during the one hour 28 minute contest and after after setting up three match points with a crosscourt backhand winner, he wrapped up the win when Del Potro overhit a forehand.
"It's unbelievable - I didn't expect this. It was tough in the first two games and then I saw he wasn't playing that well and thought 'Why not today?'," the 25-year-old qualifier commented.
Vasselin's compatriot Gilles Simon, the third seed, saw off Japan's Takao Suzuki 6-3 6-4 and defending champion Tomas Berdych, seeded five, defeated Go Soeda 6-2 6-4.
Eighth seed Lleyton Hewitt also progressed safely with a 6-1 7-5 victory over Tatsumi Ito.
"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," Hewitt commented.