For me it would be hand subtly resting on the SP101's grip while the other is casually relaxed, laying on the ground next to me.
Move your body from a sprawling i-just-got-knocked-on-my-ass position to a relaxed, lounging position. From there, talk. Calmly. If he attacked with his body first, then pulled the knife, odds are he's not really prepared to use it. He's waiting for you to fight back, to take a swing at him to galvanize him into doing something he's not otherwise ready to do on his own.
Basically, chill out. You have the gun, you know you have the gun. He doesn't. So while he's standing there flexing his vocabulary, smile disarmingly at him, reassure him that you were just trying to catch up with an old friend (IF that's indeed what you were doing

) and give him some time to blow off some steam and reassure himself of his manhood.
In the meantime, quietly squeeze the thumb strap open on your holster, get a good grip, and wait for him to make a move. By acting like you suddenly decided to lay down on the grass you make HIM the center of attention, Not yourself.
Its a poor situation to be sure, being on your back in the dirt with nothing solid to back in to, but odds are good that getting to your feet is going to put you in a much worse situation that staying low, ready to fire from prone(ish). Your going to give yourself a bigger blind spot (behind you) and your going to give him a great reason to give you some more attention.
I mean, how manly can he possibly look by yelling/brandishing a knife at some dude that's just laying on the ground, smiling, chilling. Odds are better that anyone who jumps into the scene will be doing so in YOUR defense, not in his.
Use social pressure to his ego before copper-jacketed pressure to his brain.
IMO he's some punk kid trying to look hard. He's not worth your bullet, certainly not worth several months of your life and a small fortune in court fees.
If you pull your gun, and do ANYTHING but immediately fire, you have now put him in a situation where his microscopic brain is faced with either losing face in front of hundreds of peers AND his girlfriend, or pulling what i'm sure he envisions as some slick, jacki-chan move to disarm you and kick your ass.
In the end, you end up with blood on your hands that didn't have to be there.
...and lets just say your social circle wont be setting any records either.
Its not about one-upping some stupid punk who's trying to impress his girlfreind.. its about survival.
just my $.02
...there's a certain line that comes to mind about bringing a knife to a gunfight that I will refrain from using in light of our esteemed colleagues of Italian descent..
