Combat at highest altitude

I have been a Phantom fanatic for some time but just recently, after going over her performance statistics, I’ve started to think about its maximum celing range – which is 60,000ft. That’s 18km+ just inside the stratosphere 😮
These figures are about the same for other modern aircrafts as well…

Is it possible for any fighter aircraft to undertake air to air (or even air to ground :confused: ) combat at that altitude?

Has it ever been done?