Tennis: Nadal shock is Wimbledon wake-up, says Djokovic

World number one Novak Djokovic admitted Tuesday that Rafael Nadal's shock Wimbledon exit was a resounding wake-up call as the top-seeded Serb began his campaign for a second All England Club title.

Nadal, the eight-time French Open champion and 12 times a Grand Slam title winner, suffered his worst loss at a major Monday when he was knocked out in the first round in straight sets by Belgian world number 135, Steve Darcis.

"You cannot take anything or anybody for granted. You have to be grateful for being in this position, work even harder to stay there. That's the whole point," said Djokovic.

"The sport is evolving. Everybody is getting better, getting more professional. Especially at the opening stages of Grand Slams, there are a lot of quality players who have nothing to lose really coming on the Centre Court or Court 1, playing in front of 10,000 or more people.

"Against one of the top players, you know, what can you really lose? You're going and you're playing your best tennis."

Djokovic's last loss in the first round of a major was way back in 2006 at the Australian Open.

He was in no danger of following Nadal out of the tournament on Monday as the 2011 champion cruised past tricky German Florian Mayer, the world number 34 who had had beaten in the quarter-finals last year, 6-3, 7-5, 6-4.

Djokovic goes on to face either Bobby Reynolds or Steve Johnson.