Rafael Nadal could be a threat in Paris. Djokovic will keep a close eye on him going into the clay season as the Spaniard, 30 this year, looks to put his French Open tally into double figures.
It is difficult to see where the danger is to Djokovic outside the top five. Stan Wawrinka can overpower him on a given day, but you sense Andy Murray has to win a close match between the two before he believes he can beat him on a regular basis. Roger Federer's decision to largely pass up the clay season has a "beginning of the end" feel to it, and again his best chance of winning a major for the first time since 2012 will be at Wimbledon.
Will Serena be Slammed out?
The great American Serena Williams bestrode 2015 like a colossus, although she ended up with the same amount of major titles as Djokovic.
The great unknown is whether she can summon the motivation to go for it all again after the crushing disappointment of the calendar Grand Slam near-miss at Flushing Meadows. Her body will also need to defy the onset of tennis's bus-pass age.
World No2 Simona Halep flopped at Wimbledon and Roland Garros and did not get past the semis at a Grand Slam. The No 4, Maria Sharapova, featured in a brilliant final against Serena Williams in Australia, but there are murmurs that she may not play too many more seasons.
It will be interesting to see how becoming engaged may affect Petra Kvitova, who endured a difficult 2015 while showing at times that she has all the tools to win big.
Can Andy Murray slay the beasts?
As Murray admitted, the downside to an otherwise excellent year was his record against the very best players. If we nostalgically still package them as Djokovic, Federer, Wawrinka and Nadal, he went 2-10 in those encounters.
His play at the net looked as assured as it has ever been but he will be looking to do more damage with his returning skills when it comes to facing the top players.
Will we start to see a new generation emerging?
The future for American tennis - which in the men's game has gone through a profound slump - is finally brightening. There are some excellent prospects from the US, such as Jack Sock and Jared Donaldson, yet supplanting the top players may have to wait a year or two, especially in the men's.
The younger group most worth watching come from Australia. Bernard Tomic has made strong progress but the one with the most powerful game to trouble the best may still be Nick Kyrgios.
It is to be hoped that the wild child comes closer to achieving the difficult balance between staying on the right side of the line in terms of behaviour while retaining his strong individuality and not becoming too sanitised.
Garbine Muguruza, already No 3, is most likely to upset the top order on the women's side.
