Novak Djokovic has warned that Rafael Nadal's humiliating Wimbledon defeat to Czech world 100 Lukas Rosol on Friday was a wake-up call for the game's superstars.
The world's top three men - Djokovic, Nadal and Roger Federer - have won 31 of the last 34 Grand Slam titles, a streak stretching back over seven years.
But Nadal's shock second round five-set defeat to journeyman Rosol was the Spaniard's worst performance at a major since losing to Gilles Muller of Luxembourg at the same stage of Wimbledon in 2005.
The defending champion Djokovic says you cannot take anybody for granted and you can't underestimate any opponent.
He says tennis is improving and everybody plays equally well.
Djokovic had a minor scare against another Czech on Saturday in a third round match which was played, just like Nadal's, under the Centre Court roof.
The top seed dropped the first set against 33-year-old Radek Stepanek before easing to a 4-6 6-2 6-2 6-2 win.
