In top fuel

Top fuel is a category or bracket.

(they use the most potent fuel, a nitro-based mix)

Top alcohol, funny car, pro street, these are all classifications for cars to race by.

Thanks.

But I still couldn't get it clearly.

There are different categories for automobile races. Top fuel refers to one of those categories. Antron Brown took the first place in the category of automobile racing known as top fuel.

Got it.
Thank you.

and I suspect that this is drag racing, i.e. one or more cars competing to see how fast you can cover a given distance (in the US usually a quarter mile, I'm not sure if it's the same distance in metric countries, or indeed whether this kind of racing is purely an American phenomenon) from a standing start, i.e. not moving. So the winner covered a quarter mile in 3 plus seconds, going from zero miles an hour at the start of those three seconds to 319 miles an hour at the end.

If you want more information on drag racing, here's a wikipedia article:
http://en.wikipedia.org/wiki/Drag_racing

"Top fuel" seems to mean it's running on 90% nitromethane and only 10% some sort of gasoline.

Holy ****! Back when I was following it, Big Daddy Don Garlits had the top fuel record and if I remember correctly it was just under 7 seconds, 6.97 or sum'n and around 215 mph.

Yeah, they're really burning up the road now (literally).