Novak Djokovic has claimed that a younger group of players will compete to win at the four Grand Slams in 2015.
There were surprises at both the Australian Open and US Open this year as Stanislas Wawrinka and Marin Cilic joined the sport's elite, and Djokovic is certain that the experienced players will continue to be challenged by their younger rivals.
"It's really unpredictable, it's going to be hard to say who are the clear favourites to win the Grand Slams," the world number one told reporters.
"Obviously Cilic, Wawrinka and some new names who have been winning Grand Slam titles and challenging the best and winning against the best in the big events, they have proven to everybody that it is possible.
"Now it's normal to expect that there is a new generation of players, younger players that are right at the top like [Kei] Nishikori, [Milos] Raonic, Grigor Dimitrov who have the quality to win against the best and they have done that."
Djokovic triumphed at Wimbledon in July to record success in a Grand Slam for the fourth successive year.
