Rafael Nadal has questioned the seeding process for Wimbledon that sees grass-court specialists ranked higher than those who struggle on the surface.

Unlike any of the other grand slams, Wimbledon takes players' previous performances on grass into account when coming up with their 32 seeds for the tournament.

Every other major event uses the world rankings to decide their top players, while also taking protected rankings into account for players who return from lengthy absences.

Wimbledon's decision not to follow suit has raised some eyebrows, including those of two-time champion Nadal.

"Is a controversial thing, because when you are fourth in the world it's because you deserve to be fourth in the world," he told a media conference at Roland Garros.

"When you are first, you deserve to be first. And when you are 33, you are 33 and you are not seeded.

"The only thing I am concerned with is when somebody gets injured. For example, [Juan Martin] Del Potro got a tough injury and he came back as number 700 in the world, maybe there can be an exception and he can be seeded, because his level is like one of seeds.

"But if you don't get injured, I think it is a controversial thing and is about an opinion of a few people.

"When you are in the hands of the opinion of other people, you know, nothing is 100 per cent clear. 

"The numbers are the numbers. Tennis is a fair sport, and is easy to understand. 

"When you are number one, it is because you deserve to be number one. When you are number 50, it is because you deserve to be number 50. That's all."