New York: Novak Djokovic was once the sick-note Serb whose undoubted promise was at the mercy of a variety of frustrating aches, pains and assorted strains.
But with a third US Open title within his grasp on Sunday, and having completed the career Grand Slam in Paris in June, there is little doubt that the 29-year-old world number one is now the sport's greatest warrior.
He has 12 Grand Slam titles while in total he owns 66 career crowns.
His seven-trophy haul in 2016 includes a sixth Australian Open, a maiden Roland Garros and a record 30th Masters.
There was an unexpected blip at Wimbledon where a third round exit was written off as a consequence of unspecified personal problems, now resolved.
"I'm honoured to be a part of the legends of our sport and to manage to win this many Grand Slam trophies," said Djokovic, who started playing tennis when he was four before leaving for Germany when he was 12 to escape NATO bombs blasting Belgrade.
The Serb's ability to thrive amongst the greats was never in doubt, but the size of his heart certainly was.
At Wimbledon in 2007, he retired with a back injury in the third set of his semifinal against Rafael Nadal.
He also quit at the 2006 and 2007 French Opens at the third round and quarter-final stages respectively, while at the 2009 Australian Open, where he was defending champion, he pulled out of his quarter-final with Andy Roddick citing heat exhaustion.
But at the 2012 Australian Open, nobody was questioning his courage anymore when he beat Nadal in the longest Grand Slam final of all time, a draining 5hr 53min masterpiece.
Djokovic captured the first of his 12 majors in Melbourne in 2008, but it was three years before he added his second.
He dropped gluten from his diet, his lithe physique allowing him to chase down lost causes, transforming him into the rubber man of tennis.
Djokovic also refused to accept the dominance of Roger Federer and Nadal, who were carving up majors for fun.
After leading Serbia to a maiden Davis Cup in 2010, he raced through the first half of 2011, building up a 48-1 winning run.
Only a semifinal defeat at the French Open prevented him from becoming just the third man to capture a calendar Grand Slam.
Despite that, he still finished 2011 with a 70-6 win-loss record, a haul of 10 tournament victories and year-end number one for the first time.
Back-to-back Australian Opens followed in 2012 and 2013 although the French Open remained frustratingly out of reach with three heart-breaking losses until this year's breakthrough.
Djokovic is now just five majors behind Federer's record of 17 but is the best part of six years younger than the Swiss whose last Slam came at Wimbledon in 2012.
Nadal's 14 majors is his next target.
In Paris this year, he became the first player to break through the $100 million barrier in prize money.
Djokovic is also not afraid to innovate, bringing in Boris Becker as part of his coaching team for the start of the 2014 season. He became a vegetarian this year.
Off court, Djokovic married long-time girlfriend Jelena Ristic in July 2014 and they have a son Stefan, born in October that year.
However, despite his achievements, Djokovic appears doomed never to be held in the same esteem as the clean-cut Federer or Nadal, the humble assassin.
There are those that see something a little more calculating in the Djokovic make-up.
In last year's French Open semifinals, he was castigated for taking an eight-minute medical time-out after dropping the third set against Andy Murray.
The Scotsman had lost the first two sets but the momentum was with him when play was halted for the night. Djokovic completed victory the following day.
At Indian Wells this year, he was roundly criticised for his comments on equal prize money for women.
Then, at Roland Garros, he admitted he was lucky not to have been disqualified when his racquet slipped from his hands and came within a whisker of hitting a line judge in the face.