Former Wimbledon champion Marion Bartoli believes Rafael Nadal has fallen out of the 'Big Four' following his second round exit at the All England Club.

The Spaniard crashed out of the tournament after a 5-7 6-3 4-6 4-6 loss to German Dustin Brown, which saw his ATP ranking remain at 10.

Nadal's recent lack of form has received a lot of criticism and Bartoli reckons she knows why.

"When you play against him [Nadal] the ball is 25 kilometres per hour slower, the opponent doesn't have the same difficulty to play," she told Perform.

"When he serves, he can't go beyond 115 miles per hour.

"He is exposed in every return.

"There is an [solution] and his team need to find it because it's vital for him."

Frenchwoman Bartoli - champion of the grass-court event in 2013 - is adamant 29-year-old Nadal will bounce back and believes the quality of the men's game will only increase with several players contesting for the top positions.

"There is a group of five or six players [at the top]," Bartoli said.

"There is Novak [Djokovic], Andy [Murray], Rafa, [Kei] Nishikori, [Tomas] Berdych [and] Stan [Wawrinka]. This group is at the top, top level.

"The level and the difference between the players is very small."