Rafael Nadal has admitted that he is concerned by the lack of practice he has had on grass ahead of the Wimbledon Championships.

The 27-year-old has enjoyed a successful comeback from injury this year, winning seven out of the nine tournaments that he has entered.

However, the Spaniard has voiced fears that he may be unprepared for the All England Club.

"I am going without having trained on grass and it is a surface that you have to understand, learn once again and know how to play the points and focus in the right manner in certain situations," Nadal is quoted as saying by Sky Sports News.

"I have gone practically two years without playing on grass because last year I arrived in very bad condition due to the knee, just as much in Halle as at Wimbledon."

Nadal, who has won Wimbledon two times, suffered a surprise second-round defeat to Lukas Rosol in last year's tournament.