Andy Murray Enough said about Murray in 2016 - that will be his attitude, if nobody else's. The Scot turns 30 in May and he will know that there are, at a maximum, four more years for him to be seriously competitive and cement his legacy.
This starts with him going into this month's Australian Open as clear favourite for the first time.
If Murray never wins this title, it will be a massive disappointment for him, as he has shown so often how the conditions suit him.
He has made the slightly dubious decision to start the year in Qatar, potentially leaving relatively little time to prepare in Melbourne.
The big question, however, is how he handles being the hunted, rather than the master of the hounds.
Novak Djokovic On the evidence of the second half of 2016, the great Serb is on something of a voyage of self-discovery, as interested in cosmic self-awareness as in winning titles.
Becoming more spiritually minded has not done wonders for his tennis, and by the end of this month, we will have a much clearer idea about whether he has managed to reset his career, or whether some form of serious burnout has set in.
His record at the Australian Open is extraordinary, having won five of the past six editions.
Should the post-Paris Djokovic turn up and fail to win, it will suggest that his era is properly on the wane.
Should he collect another Melbourne title, then another triumphant season beckons. Selling stock in this remarkable athlete may yet turn out to be a fool's game.
Serena Williams Actions did not always quite match words for the American last year.
On the one hand, she is inclined to breathe fire at anyone who suggests that the women's game is not the equal of the men's.
Yet she still believed that, in her 35th year, one player (herself) would be able to dominate by playing what was, effectively, a part-time schedule.
The onset of nagging injuries may also be a factor. However, she does have the incentive of wanting to overtake Steffi Graf's total of 22 majors, and her armoury remains so superior to others that she ought to come good for at least one of them.
Maria Sharapova Whatever one now thinks of Sharapova, the women's game has missed her, and her absence from the tour after March's jaw-dropping doping admission was a severe blow.
Having done the crime, she will have done the time by the start of May and there is already a wildcard on the table for her at the Madrid Open that begins on May 7.
Given the dearth of proven campaigners on the WTA Tour, she ought to make fairly brisk headway from a non-existent ranking, and there are few more determined people.
Roger Federer For many, the Swiss master playing only 28 matches in 2016 will have been one of the worst aspects of the season.
He called it quits after Wimbledon to deal properly with a knee problem but reports from his camp suggest he is champing at the bit to come back.
Federer begins the season in the unaccustomed position of being ranked 16, and if he does not fare well at the Australian Open, he will drop into the thirties, and could be unseeded for many events.
The best bet is that he will, sooner or later, move back towards the top 10, but that another major will be beyond him.
It will be interesting to see how his motivation holds up when he is losing regularly to players from outside the elite few. That may not happen for another year.
Rafael Nadal The Spanish clay-court genius still managed to finish in the top 10, despite finishing the season early and missing a large chunk of the summer.
His palatial academy in Majorca is now open and one wonders if he is starting to think of life after the grind of the circuit, which he has been gracing since his mid-teens.
It could be that the restoration of his old confidence will be just as important as staying healthy.
Jo Konta Backing up her breakthrough year of 2015 - and then some - was a huge achievement for Britain's No 1, seemingly impervious to the pressures associated with defending points.
There will be no let-up in the demands, however, and she faces a testing time next month when she returns to Melbourne, scene of her semifinal appearance of 2016.
This will have to be done without the coach who did so much for her, Esteban Carril, and in the wake of the death of Juan Coto, her sports psychologist.
Jo Konta. Photo / photosport.nz
It looks as if she will replace him with Belgian Wim Fissette.
He is only 36 but has an impressive list of coaching credits to his name, including Kim Clijsters and Victoria Azarenka.
How this works out will be crucial to her chances of remaining around the top 10, or even progressing with a game that now has so few holes in it.
Nick Kyrgios There is one thing on which the cognoscenti agree when it comes to the enfant terrible of tennis - he has all the raw equipment to achieve anything he wants in the sport.
Finishing a year truncated by a suspension for tanking at No 13, and with two ATP titles to his name already at just 21, shows how much potential there is for one still trying to figure himself out.
As part of his sanction, Kyrgios has been working with a psychologist, and played in the unofficial Asian tennis league during the off-season.
While there will be lots of speculation about if he is a reformed character, the key may be something rather more prosaic: whether he can put in the amount of work to sustain himself in peak fitness and health.
