Federer and Nadal’s rivalry has shown no signs of slowing up with both men enjoying a stellar year.
Roger Federer won the Australian Open in January and, after missing the French Open, claimed the Wimbledon title with victory over Cilic in July.
Nadal, meanwhile, won the French for the 10th time and also won the US Open, defeating Kevin Anderson 6-3, 6-3, 6-4 in the final.
And, while the Spaniard will end the year as the oldest No1 in history, Cilic thought Federer started the year in better form.
When discussing the year the two men have had with Novak Djokovic and Andy Murray struggling, he said: “They were part of the tour the first six months and they, both Andy and Novak, were not at their best form.
“They were struggling at the beginning of the season – the opportunity opened up for both Roger and Rafa and for most of us.
“We all needed to be ready to take that advantage and I would say that Roger came out of the blocks a little bit better than Rafa.
“Rafa started to get his rhythm on the clay court season and did tremendously well and kept form unbelievably since then.
“They played the best season on the tour and they deservedly are at the top.
“Next year is obviously going to be a big challenge to take them down.
“We’re going to see how Andy, Novak, Stan, Kei and Milos are going to come back.
Federer and Nadal are both set to be part of the ATP World Tour Finals, which kicks off on Monday.
Nadal has been battling a knee injury but the Spaniard appears to be winning that fitness battle.