Unless the centaur is particularly incompetent, the human's best chance is probably for a lucky break to occur which aids him.
First, note that this question is constrained in such a way that it gets rid of what would be the humans best advantages in other situations. This assumes a dirt circle flat terrain, while humans are remarkably good at handling more varied and rough terrain.
This also assumes no weapons where humans are very much tool using creatures. Of course, the Centaur presumably is as well, but the lack of tools will hurt the human more here.
We are also assuming a one-on-one fight were humans are pack animals. Of course, so are horses and presumably centaurs, but with the size differences this hurts the human more.
The one constraint that does favor the human is that we are assuming a "little" dirt circle which will constrain the centaur's ability to charge, but unless we are talking truly small area that is not a huge advantage.
Others have suggested tiring out the centaur. Tiring our your opponent is often a good idea in a fight, but a centaur is not likely to be particularly susceptible to that, no more so than the human is. Humans are endurance predators, but horses were not particularly high on our list of prey and horses are also built for endurance. Unlike many other animals, they sweat much like humans do and have good lung capacity. In human vs. horse marathons, the horses (with riders) win more often than the humans. https://en.wikipedia.org/wiki/Man_versus_Horse_Marathon
(We could of course argue that the centaur has less endurance than a horse due to being restricted to a much smaller human-sized nose rather than a horse sized nose, but I assume if we are positing the existence of centaurs at all that we are hand-waving that away. If we aren't, then tiring the centaur out is probably not only going to be plausible but incredibly easy to the point of the human winning by making the centaur exert itself slightly and then falling over under the many anatomical issues a centaur would have...)
It might also appear that the humans greater agility is an advantage. And it is, but not a big one under these constraints. It would be a bigger advantage in a more varied environment like an urban area or dense forest. If you ever watch barrel racing competitions, horses can be quite nimble.
Getting on its back is also a good idea, but much harder than it sounds. Its already hard to mount a horse that does not want to be mounted. (Without benefit of either mounting blocks or stirrups, it can be a pain to mount a horse that isn't fighting...) This scenario removes many of the common strategies for achieving it by assuming flat terrain and giving the human neither teamwork nor tools. And then, depending on the particular interpretation, the centaur can probably reach much or even all of its back to swat the human off.
If you are writing a story and want the human to win under these conditions, your most plausible scenarios are either the centaur is not a competent fighter and makes a mistake the human can exploit or the human gets a straight lucky break such as the centaur tripping.
Of course your human will have various strategies it can try to use, such as going for the legs, staying on the flank (sides) but the match is heavily stacked in the Centaur's favor. Cavalry was immensely effective on the battlefield up until motorized vehicles for a reason. Of course, cavalry was often defeated but that tended to involve weapons, teamwork, and constricted or unfavorable terrain, all of which have been essentially assumed away in this question.