Tim Henman refused to use a possible illness as an excuse for his Madrid Masters defeat to Ivan Ljubicic. The top seed took a blood test on Wednesday to discover the cause of his fatigue, but he said his 6-4 4-6 6-2 loss was down to his own poor play.
"From the waist up, I feel fine. I don't feel ill, but I just don't have any energy," he said.
"But I was hitting the ball poorly, it's as simple as that. I was erratic - every game was a struggle."
Henman came down with a similar energy-sapping illness just days before the start of this year's French Open, but he went on to reach the semi-finals.
However, the 30-year-old's latest loss is a setback to his hopes of reaching the end-of-year Masters Cup.
Henman is currently sixth in the Champions Race, with only the top seven certain of making the elite eight-man field. Henman is next due to play at the Swiss Indoors in Basle, which begins on Monday.