The Swiss bagged a historic win over Marin Cilic back in January.
And, in a topsy turvy year for the superstar, Castle is adamant that the triumph was a special occasion.
When asked by The Metro what his 2018 highlight has been, Castle said: “Roger Federer winning a 20th grand slam at the Australian Open.
“The Swiss claimed the world No.1 ranking as a 36-year-old this year.
“No one older has ever topped the rankings.
“I hope he goes on forever.”
Despite his impressive start to the season at the Rod Laver Arena 11 months ago, Federer has struggled in recent weeks – Basel aside.
He was dumped out of the ATP Finals in London by Alexander Zverev after a semi final showdown last week.
Zverev went on to win the competition after beating Novak Djokovic in the final.
As well as a cool £2m in his back pocket, Zverev also secured 1,300 ATP Ranking points to finish the year as world No 4.
“I really can’t describe it,” Zverev said.
“It is the biggest title I have ever won.
“Firstly I want to congratulate Novak and we may never have seen the tennis he has played in the last few months before.
“He barely lost a match but thankfully he did to me.”
Federer, meanwhile, says he wants to work on getting forward to the net more often in his preparations for next season.
“As far as tennis is concerned, it would like to find its way back to the net more often,” the Swiss said.
“And of course I want to whip the forehand right again.
“Then there is the condition training with Pierre (Paganini).
“Of course, deciding whether to play sand or not has an impact on the training program.”
Federer is in Dubai over the coming weeks.
His first event of the year will be at the Hopman Cup in Australia.