i came across a problem while reading newspaper.there was a title read,"Cavendish: On best form for two years ahead of Tour de France".that sounded a little weird to me,because i felt that"in (whatever) form" should be the right combination.and i googled,the result showed in favor of my opinion,like"David Silva: I'm in best form of my life at Manchester City","Phelps in 'best form since 2008'".but i still want to make it clearer,which one is correct/sounds more natural to you?are they interchangeable?

I will be grateful for any help you can provide.

Anonymous,"Cavendish: On best form for two years ahead of Tour de France"
This seems to be a headline and not a sentence.

"He's on his best form" is less common that "He's in his best form," but it's acceptable, in my opinion.

Exactly what your quote means with respect to time factors is open to interpretation, unless you happen to be familiar with this particular sportswriter's style.

- A.
thank you for your answer!