"Career-defining" is sometimes an overused term. Most commonly it refers to a moment when one's career reaches its definitive moment, i.e. typically a major triumph or success of some kind.
More unusually, as in this context, it is used to describe a moment that defines not only one's career by success, but by character.
Roger Federer and Rafael Nadal have had these kinds of career-defining seasons. Federer had his typical career-defining year in 2006, when he topped the charts in just about every department possible and blew the lid off of what was acceptably brilliant.
But his real career-defining season was arguably 2007 or 2008—years in which he enjoyed success, but only at the expense of much pride and normality.
For Nadal, his typical career-defining season was very likely that of 2008, when he won both Wimbledon and the French Open and ousted Federer as the No. 1 player. His real defining year, however, was arguably 2010, when he had to endure mental hurdles to triumph in big places.
There is no doubt about when most people would say Novak Djokovic enjoyed a career season—2011. Winning three majors, claiming the No. 1 ranking and compiling a 70-6 win-loss record are among the achievements most tennis players try to reach in their entire careers, let alone over a single year.
When, however, will his biggest test come?
Reaching the top is always a tough road in tennis. Rafael Nadal proved that, having to wait nearly four years at the second spot before usurping Federer. Andy Murray proved that at the U.S. Open just recently; after four losses in Grand Slam finals, he won his first Grand Slam title on his fifth try. Staying at the top, having reached star status, is the next step, and quite often has proven harder than most greats could manage.
The essence of a real career-defining season should be that of triumph; not per se, but triumph in the face of hardship and adversity.
So often an annus mirabilis needs to be qualified by an annus horribilis. Roger Federer proved that in 2008-09, and Nadal in the same years and in 2010-11. Has Djokovic in any way encountered a similar pattern in 2012?
In a sense he has, and a single stat illustrates this. At this point last year he had already amassed 10 titles; this year, only three. Moreover he was 10-1 in finals in 2011, but this year already is 3-5.
A year with a Grand Slam title (the Australian Open) and two Masters series titles (Miami and Toronto) cannot really be called an annus horribilis, just as it was far-fetched to do the same to Federer's 2008 or Nadal's 2009/11 (they both won slams and Masters Series titles). Yet in the context of his so-called "career-defining year" it is a quantum leap in tennis terms.
Several things have gotten in the way of Djokovic's repeat ambitions in 2012, and they have not always been from him alone.
One has been Andy Murray's forehand and psychological sturdiness. He pushed him in the Australian Open semis, and finally toppled him at New York. That slightly delayed forehand is now being hit with authority and it has worked well.
Then, of course, there is the ever-durable Roger Federer, who has played well practically all season, gaining points on Djokovic all year until Wimbledon, when he stepped up in their semifinal clash and shifted the power in his direction for that moment. His dominance of the Serb endures for now—most recently at the Cincinnati final, where he handed Djokovic his first bagel in their rivalry.
Yet surely, something has come off ever so slightly from the Djokovic freight train. His consistency has dipped, his ability to reach top form in tense moments rusty, even his fitness not quite at the standards of 2011. It's all very hard to define, really—Djokovic has been pretty much the same player this year and last. But such is the nature of tennis—winning and losing being parts of the experience.
Will he turn around this 2012, and pass the tests it has brought him? His remaining goals for the rest of the year—the year-end No. 1 ranking, the Barclays World Tour Finals, a few wins over his big rivals—would that a real career-defining season make?
Perhaps not. But it will have been a season when he learned that, with great success there inevitably comes great loss.