Nadal has struggled in the past few seasons with injuries and a lack of consistency.
But he was played some of the best tennis of his career this year, winning both the Australian Open and US Open.
He claimed the latter last night with a 6-3, 6-3, 6-4 win over Kevin Anderson at Flushing Meadows.
The Spaniard has now won 16 Grand Slam titles and is just three behind Roger Federer.
And former Australian tennis player Cash revealed he thought the world No. 1 should’ve considered calling it a day before he even hit a ball this year.
Cash said: “There has been quite a few times over his career where we’ve thought ’Is he going to keep going? Is this the end?’
“I saw him right at the beginning of the year at the first tournament in Abu Dhabi and when I knew that he was coming to play that tournament in the back of my mind I thought ‘I don’t want to see Rafa go downhill, maybe he should retire’.
“His shots were going all over the place in the last two years, they were short, they were deep, his serve had fallen away, he’s getting injured all the time – you don’t want to see a champion fade away like that.
“But when he came out to Abu Dhabi and I saw the first match and I thought ‘Oh my goodness’.
“He hit the ball 25 per cent harder than I’d ever seen him hit, he was serving big and I thought, wow – this is a fantastic effort.
“To win two Grand Slams and then be a point or so away from winning a third was phenomenal.”