How about these:

Goran Ivanisevic entered Wimbledon 2001 as a wild card and won the title against Pat Rafter who had been ranked as world no. 1 (albeit in 1999 for one week, but it shows his calibre). I was rooting for Goran that year and predicted him to win (not that it has anything to do with this argument, I just felt like sharing it LOL) - yes, a wild card!! Pretty amazing achievement on his part I would say.

Jimmy Connors beat John McEnroe (defending champion) in 1982. Jimmy was in his thirties - not old per se but old in sporting terms. Awesome!

Maria Sharapova beat Serena Williams (defending champion) in 2004 after only her second ever appearance at Wimbledon.

Whilst I agree that it is highly unlikely that Federer will be beaten, (five times in a row is a remarkable achievement), as I said before, I never say never.