Washington: Fifth-ranked Tomas Berdych, hoping to crack the top four in men's tennis for years, says Novak Djokovic and Rafael Nadal have resumed their stranglehold at the top despite Stanislas Wawrinka's shock Australian Open title.

The 28-year-old Czech, seeking his 10th career ATP title as the top seed at this week's ATP and WTA Washington Open, said Monday that number one Djokovic, coming off a Wimbledon title, and second-ranked Nadal, who won the French Open last month, combine with 17-time Grand Slam winner Roger Federer to make the going tough for challengers.