That is beyond dispute. This 33-year-old Belgian enters tomorrow's London Marathon as the fastest man in the field and the only man in history to have run two marathons in less than 2hr 8min. But for all his ability and achievement, today is going to be a pig of a day for him, because it always is.
"The day before the race is more heavy than the race itself," he said. "When you are on the run, you are on the inside. You let the dogs go away..."
He welcomes the felicitous image with a puckish grin which transforms his wan features. Rousseau looks like a man who has suffered for his art - pale as a clown, pin-thin - but his droll humour keeps on breaking through to transform the picture.
It was evident earlier this week as he explained his controversial decision not to race in Atlanta because of the hot and humid conditions. "Some people cannot understand why, when it comes to the Olympics, I say `no thank-you'," he said, with a grin and a flip of his hand. It is not controversial with him, because he has thought it through as he appears to think everything through in his life.
After several distressing experiences of running in hot conditions, he decided that, if he was not happy, he would not race. He has a contract with his home federation stating that he does not have to represent his country if temperatures exceed 18C (64.5F). If anybody doubted the seriousness of his position, he underlined it last year when he walked away from the start line of the Rotterdam Marathon, and a guaranteed appearance fee of $100,000 (pounds 66,000) because temperatures had risen to 70F. Had he run only a few steps, he would have received his money in full. Odd Rousseau may be, but no one could ever accuse him of being cynical.
"It is a physiological thing for me," he said. "I think it is easier for me to run 2.07 in the cold than a 28min 40sec 10,000 metres in hot conditions."
There is no escape clause for him in tomorrow's race, but in London such a clause is hardly necessary. With the two Mexicans, Dionicio Ceron and German Silva, in the field, along with Domingos Castro of Portugal and the strongest British contingent in recent years, the temperature will not be uppermost in Rousseau's mind.
"I have no way to win the Olympics, because I cannot run in hot weather and, if I go to the Olympics, I go there to do something," he said. "I prefer to win the big marathons."
He is cautious about his prospects of adding the London title to his wins in Rotterdam, Reims and Brussels, having undergone an Achilles tendon injury in January. Asked if he was in good shape for tomorrow, he actually considered the question rather than giving the stock reply. "Yes," he said, "I'm not bad."
Rousseau is a thoughtful, solitary character. He lives alone in the small village of Masnuy-St Jean, near Mons, and enjoys gardening and nature photography. The latter interest has been indulged on his rest days when he has been training at altitude in the Pyrenees.
"I can do mountains, and wildlife. But I do not have enough time to do it properly while I am running. I will do it afterwards for pleasure."
The question of when his photography will begin in earnest is an open one. But he does acknowledge that if he is to break Belayneh Dinsamo's eight-year-old world record of 2:06:50 - 30 seconds faster than his best - he will have to do it in the next year or so.
Like time's winged chariot, ever at his back he hears Gebreselassie and Tergat hurrying near. The Ethiopian's world 10,000m record of 26:43:53 is 40 seconds faster than his best, and converts - in purely arithmetic terms - to a 2:04 marathon time.
Rousseau, however, feels Haile Gebreselassie's step is too high for marathon running. The recent half-marathon world record of 58:51 set by Kenya's world cross-country champion, Paul Tergat, can be converted to a time of around 2:05, and Rousseau feels that if Tergat turns to the marathon in a couple of years, he could be running times beyond the reach of Europeans, and even Mexicans.
Rousseau makes these calculations with a twinkle in his eye, and you wonder how much irony is in his delivery. The marathon is an unpredictable event, and as he remarks himself, "men are not machines".
Paradoxically, the worst moment of Rousseau's career occurred when he set his personal best of 2:07:20 at Berlin last year, because he was beaten by Kenya's Sammy Lelei, who ran 2:07:02, the second fastest ever. Rousseau led after 39 of the 42 kilometres. "I lost a lot in a few kilometres," he said.
Rousseau, however, is a driven man. "Running is a drug for people like me," he said. "If you cannot get your adrenaline going, you are sick. When I cannot run, I feel heavy, I feel fat." This man is not fat. He will be even thinner by noon tomorrow.Reuse content