Roger Federer defeated Marin Cilic at the Australian Open on Sunday to claim his 20th Grand Slam title, making him one of only four players to reach the figure over their careers.

His battling five set win over one-time Grand Slam winner Cilic ensured that his renaissance continued in the latter stages of his career, having won three out of the last five major titles.

Though most people within the sporting world would make a case for Federer being the greatest tennis player of all time, certainly within the men's game, the Swiss is currently 155 points behind current world no. 1 Rafael Nadal in the men's rankings.

As Federer continues to add to his trophy haul, former Wimbledon champion Pat Cash now thinks that he has one more goal that he wants to achieve before he retires from tennis.

"I think there's probably one last thing that he'd really like to do," Cash said on Eurosport via Express.

"That is to be the oldest world No 1. I know that for a fact that he wants to be the oldest No 1."

Federer has previously said that he is content with enjoying his success that few expected would come so late in his career.

The 36-year-old's win on Sunday puts him within reach of world number one Nadal, and if results continue to go his way in the next few months, he could overtake the Spaniard and break Andre Agassi's record of being the oldest world no.1 (33 years, 131 days).

Federer had originally missed the opportunity to cap a remarkable 2017 as the world number one when he had to pull out of the Paris Masters in October last year.

However, Cash believes that the spot is still firmly within Federer's sights.

"He had to defend his title to get that, of course he's got some other big titles coming up. Miami, Indian Wells, Wimbledon of course where he's always going to be the favourite.

"Rafa's just got to drop off, which he has, so he's going to edge closer to that. I think that will always be 'well, I've done everything now'."