The Oxford English Dictionary already includes an entry for come on: come on! the imperative is used as a call to urge someone to advance towards or to accompany (the speaker), or to proceed with anything; esp. used as a challenge or call of defiance. In years to come, perhaps the editors of OED will add the phrase come on Tim! as a specific example of this phrase in use. The entry would read come on Tim!" the imperative is used to urge a professional sportsman to advance towards an unrealistic goal that is beyond them, esp. used as an ironic call of defiance when facing a delayed but inevitable defeat. (British colloquial, Wimbledon).
Nevertheless, I have already been rallying to this call this summer. Tim Henman defeated 25th seed Carlos Moya in the first round on Tuesday, squeezing out the former world number one 13-11 in the final set. When Tim does retire in a year or two, we will miss all the phenomena associated with his decade of classy grass-court tennis: Henman hill, Ariel washing powder adverts, Timfist to celebrate a point, and Sue Barkers will this be Tims year? catchphrase. So come on, sportsfans, one more year: come on Tim!