Think back to 2010.
Rafael Nadal had just completed his career grand slam with victory in the US Open and looked to be on his way to superseding Roger Federer as the greatest player of all time.
Fast-forward three years it looks increasing unlikely that Nadal, with 11 grand slam titles, will overtake Federer's tally of 17.
Far more concerning for Nadal, however, is his struggle with injury. Last week, he made his first competitive appearance for seven and a half months at a low-ranking clay-court tournament in Chile. After the Spaniard's exit from Wimbledon last summer at the hands of world No.100 Lukas Rosol, Nadal revealed that he had partial tears in his patella tendon, for which the rehabilitation process required a prolonged period of rest.
Now, with his recovery complete, Nadal is on the comeback trail, and he reached the final in Chile. His most encouraging match was beating Jeremy Chardy who, at No.25 in the world is certainly accomplished, but whose level is some distance below the top 10's.
So how easy will it be for Nadal to return to his old form? He was always going to be particularly susceptible to injury due to his overtly physical style of play.
Nadal raised the bar with regards to fitness levels. Every other player on the tour had to improve their fitness if they wanted to compete with him. But coming back from his most recent lay-off will prove to be the biggest test of his career so far.
Nadal will face several challenges over the next few months. Firstly, tennis has moved on since he made his last grand slam appearance. Federer, Novak Djokovic and Andy Murray all won major titles during Nadal's absence and Djokovic and Murray in particular appear to have opened a sizeable gap between themselves and the rest of the players on the tour.
If Nadal is to add to his tally of major titles then he must not only return to the level he was playing at before his time off, but he must improve upon it.
Secondly, Nadal is not a kid any more. He will turn 27 this year and, as every athlete knows, the older you get, the longer you take to recover from injury. Of course, there is no exact science when it comes to rehabilitation but an injury such as his will usually respond well to physiotherapy.
And, encouragingly for Nadal, he has a favourable track record in recovering from injury. In 2009 the Spaniard was forced into a 10-week lay-off with a similar problem and he returned to win grand slam titles and reclaim the No.1 ranking.
But that was then and this is now. Often, it is the mental rather than the physical scars which are the hardest to heal. For any athlete to return to the level that they were at prior to their injury, they must eliminate any semblance of doubt.
Nadal has said that he is in pain almost all of the time, and that he must learn to manage and cope with it. If there is even a tiny part of an athlete worrying about their injury, and of the ability of their body to give 100% effort, then they will not be able to perform to their full capability. While Nadal's comeback in Chile was a step in the right direction, he has a long way to go. His knee appeared to emerge unscathed from his four singles matches, but that was on the most forgiving surface of them all – clay.
The real test will come next month when he is scheduled to play the Masters 1000 events in Indian Wells and Miami on the brutal surface of hard court. There will be enormous goodwill towards Nadal in his attempt to reclaim his position at the top of the game.
It will certainly be intriguing to see if he manages it.