I would tell him "Sachin,you are the greatest of a whole era.There is no advice I can give you regarding batting.But,you have to forget everything about the critics,the records and your status.Just take one match at a time forgetting the century.We all know that when you play with a free mind you can do anything.Not just a hundred you can even score a Double hundred!"
Don't think about the 100th hundred. The day in which you'll forget about it, you can make the 100th Hundred on that itself. So please forget about the century. You'll play another 3-4 year after getting that hundred, that too better than ever in your carrier, So please ignore all sort of comments about the 100th century.