The time on Valentino Rossi's T-shirt signified the number of world titles this flamboyant megastar has achieved, but such is his talent that we may be getting on for midnight by the time he finally hangs up his leathers. For the moment it is 8 o'clock with, like old times, the Italian refusing to take the easy route to the top and triumphing in the Japanese Grand Prix.

The celebrations were typically colourful. Rossi dismounted his Fiat Yamaha on his victory lap and sat down with a man dressed in a suit at a desk positioned by the side of the track. The new champion said this was “a notary” to “make it official”, but given Rossi appeared to hand over his credit card, many felt the subtext was a dig at the taxman, who took £26million from him this year. It was the only issue left in any doubt. Five successive race victories mean that Rossi has completed a remarkable comeback from two barren years where intimations of mortality were sown.

“It was a crap season in 2007 and then when I heard about the deaths of Colin McRae and Norick Abe [two of his motor sport heroes] I thought, 'This is the worst year ever,' ” he said. "But losing has made me grow up. I thought I was a little bit unbeatable, but I've come back stronger. This has been the hardest championship of all.”

It is proof of Rossi's maturing that he has opened up an unassailable lead with three races remaining. There was a different winner of each of the first four grands prix, but Rossi has been an unstoppable force since he beat Casey Stoner in one of the greatest duels of all-time at Laguna Seca in the United States in July. After that, Stoner's title defence faltered in the face of two crashes and a broken bone in his wrist.

The Ducati rider acknowledged his fallibility with good grace. “It's not something that's come as a shock,” the Australian said. “Valentino has ridden an amazing season and I've been disappointed with myself. I made two mistakes that cost us the championship.”

Stoner will be back for more next season, when he will have the added motivation of a fast team-mate in Nicky Hayden. The move he made to overtake Dani Pedrosa, on the leading Honda, showed how on the edge he is riding, the raised hand a friendly gesture “because it wasn't a friendly pass”. Both will need to step up to beat a man seven years their senior in what will be a landmark year after the Grand Prix Commission decided to introduce a single tyre manufacturer for safety and financial reasons.

Bridgestone and Michelin will find out on October 18 which is successful, but Rossi suggested he would be on the former, fuelling speculation that the deal is done. Those riders blighted by Michelin tyres this year will be glad to see the end of the annual tyre war.

Rossi has not won at Motegi since 2001, the year he clinched the first of his six elite titles - the tally of eight includes 125cc and 250cc crowns - and it took half a great race to seal this win. Jorge Lorenzo, his Fiat Yamaha team-mate, was on pole position, but Stoner led at the first turn. Rossi was fifth and, once Stoner had got the better of Pedrosa, set about tracking the defending champion. He got by with ten laps to go and never looked troubled thereafter.

Rossi revealed that “a strange meeting” with Masao Furusawa, Yamaha's technical chief, after the last race of last season in Valencia, launched the title campaign. Unnecessary changes had been made within the team, Rossi said, and Furusawa listened. “I made some mistakes,” Furusawa said. Yamaha also upped their game. “The bike is just a tool but the rider could hardly manage it in 2007,” Furusawa said.

Where Rossi exudes class is in refusing to play the percentages. He did the same in 2004, wrapping up the title by exchanging the lead with Sete Gibernau, of Spain, three times on the last lap in Australia. Rossi needed to finish only second but his pride preferred to risk a fall. After a year of renaissance, he has become only the second man to regain the championship after a two-year gap. The other was Giacomo Agostini, whose record of 68 victories he has now surpassed. “I have ridden the best in my career this year,” Rossi said. “But next year is another story.”

Hence, he has remained in Motegi to test the 2009 bike today. “I am worried about my headache and so might fight for an afternoon test,” Rossi quipped. If he plays as hard as he works, then it will have been quite a party.

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