Djokovic (right) is the form player of the season so far
Rafael Nadal believes he will lose his world number two position to Novak Djokovic by Wimbledon at the latest.
The Spaniard lost valuable points when his defence of the Rome Masters title ended in the second round, while Djokovic went on to lift the trophy.
"The logical thing is that he goes past me here in Hamburg, or at Roland Garros or Wimbledon," said the 21-year-old.
Nadal was hampered by blisters in Rome but he said: "I've had a few days off, and I'm much better."
Earlier in the season, Nadal had an outside chance of overtaking world number one Roger Federer, who had a poor start to the season by his high standards.
However, Nadal now finds himself with a stack of ranking points to defend from now until Wimbledon and any slip could see Djokovic move up a place.
The Serb has been the form player of the season so far, winning the Australian Open as well as the Masters Series events in Indian Wells and Rome.
"He's a great player and he's doing things very well," said Nadal of his Serbian rival.
"If I'm number three I'm number three. When it happens I just have to accept it and fight to get the position back."
Nadal, the world's best player on clay, has complained bitterly about this year's schedule, which has three clay-court Masters Series tournaments, plus Barcelona, crammed into four weeks.
The Spaniard successfully defended his titles in Barcelona and Monte Carlo but saw his 17-match unbeaten run in Rome ended in the second round by compatriot Juan Carlos Ferrero.