Ferrari's Michael Schumacher took his decision to quit Formula One over two months ago and said that it was a desire to go out while at his best that had made his mind up.
Schumacher, the most successful driver in Formula One history with seven world titles, announced his decision to retire at the end of this season shortly after winning the Italian Grand Prix on Sunday.
But he said he had made his choice after the Indianapolis Grand Prix on July 2.
"I decided after Indy and after Indy I informed the team," the 37-year-old Schumacher told a post-race news conference.
The German, who delighted the home crowd with a win that took him within two points of leader and defending world champion Fernando Alonso, said he had proved he was still competitive and that was a major factor in his decision.
"I don't want to go away while I am over it - that is one of the reasons why I am leaving now," said the German.
"You need all your energy and motivation and strength and when you are getting older it doesn't make that any easier," he added.
Schumacher, who picked up his 90th race win on Sunday, said the contractual situation of teammate Felipe Massa had contributed to the timing of his decision.
The German said that after Ferrari had made their decision to sign Finn Kimi Raikkonen from McLaren for next season, Massa "needed to know my plans so that he could negotiate".
Massa has extended his deal with Ferrari until 2008.
Schumacher denied that last season's poor performances from Ferrari, with just one victory for the German, had led him to quit.
"No. We learnt a lesson last year and I think next year they will have a car which is competitive," he said.
Schumacher has three Grand Prix races left before he bows out but said that his last ever victory lap on the Monza course, always packed with Ferrari fans and the oldest Formula One venue, was poignant.
"It was obviously pretty emotional to take that lap, talking to the team all the way. It was emotional on the podium and it finished well with the big crowd giving me all their feeling - it was overwhelming," he said.
There was a sign of that emotion getting to the normally controlled Schumacher during the news conference.
Asked what he would miss most about the sport when he was gone from competition, Schumacher remained silent for several seconds and after taking a deep breath said only "Excuse me".