Women’s tennis in 2016 could follow a predictable path that sees runaway world No. 1 Serena Williams win three or four of the Grand Slam titles, but there are several variables in place that suggest it could be a much more uncertain year.

The first factor is Serena herself, and the growing questions about her fitness and her motivation. She is currently struggling with an inflamed knee, which is disrupting her Australian Open preparations at the Hopman Cup in Perth. Worryingly, it is the same injury that forced her to pull out of her Indian Wells semifinal against Simona Halep in March and, although she has refused to confirm anything, it may also have been what encouraged her to miss the 2015 Asian swing. Given the apparent unpredictability of the problem, it is impossible to know at this stage what sort of physical shape Serena will be in when the action begins in Melbourne on January 18.

There are also some doubts about her mental shape after she came so close to securing the holy grail of tennis – the calendar Grand Slam – only to lose to veteran Italian Roberta Vinci in the US Open semifinal. Serena’s body language during the tournament suggested she believed the title was hers to lose, and the calendar Grand Slam was well within her reach, so missing out will definitely have had some effect on her psyche going into 2016. Could it have sowed enough seeds of doubt that she might falter at crucial points in matches? We shall see.

The second factor is the form of two of the WTA Tour’s most consistent performers – Maria Sharapova and Agnieszka Radwanska. Both players could have exceptional years, but there are compelling reasons why each might not.

Sharapova was at the peak of her fitness and played superbly in the first seven months of 2015, but, considering injury kept her out of action for the next three months, can she be really be expected to play at her absolute best from the outset this year? She certainly has the mental strength to achieve it, but no tennis player is immune from dips in confidence. If the Russian falls early at the Australian Open, where she reached the final in 2015, she may lose some of her self-belief as well as ranking points.

Radwanska scaled the highest summit of her career when she won the WTA Finals last November, and this should act as the perfect stepping stone for an excellent 2016. However, she can be a streaky player – unplayable for a tournament or two and then easy to beat in the next – and will be eager to get off to a good start this year. In 2015, she started poorly, going out in the fourth round of the Australian Open to Venus Williams and then losing seven of her next 13 matches. Radwanska did not really recover until the grass court season began in June. If 2016 is to be the spectacular year the Pole undoubtedly wants it to be, she must start on a high so that she does not lose the momentum and self-belief she gained in Singapore.

The third factor is the young stars of the WTA. Simona Halep, Garbine Muguruza, Karolina Pliskova, Belinda Bencic, Madison Keys, and Elina Svitolina have all broken into the world’s top 20 before their 25th birthdays and look to have every intention of staying there for a long time. Muguruza and Halep are definitely the most likely 2016 Grand Slam winners among them, but all of these women are impressive in their own ways, and it would come as no surprise to see all of them making regular quarterfinal and semifinal appearances at the year’s major events, and some will almost certainly win smaller events. This year may be a bit too soon, but if you’re looking for an outside bet for a Grand Slam title in 2016, go for either Keys to win the Australian Open title after her impressive run to the semifinal last year or Bencic to showcase her expertise on grass by winning the Wimbledon title. It would be very exciting to see either of these outcomes actually occur.

The fourth and final factor is the WTA Tour’s most unpredictable players, a list that includes Petra Kvitova, Venus Williams, Ana Ivanovic, Victoria Azarenka, Sabine Lisicki, and now Eugenie Bouchard. Any one of these players is theoretically capable of winning a Grand Slam title, but it is so difficult to anticipate what they are going to produce from one day to the next that it makes it impossible to back any of them with any certainty. In Venus’ case, this is not her fault because her struggles with Sjogren’s syndrome effectively sap all her energy from time to time. It is also conceivable that Azarenka will regain the consistency she had before her year-long foot injury, so her presence in this list may only be temporary. Unfortunately for the others, however, they are here to stay, since their form often fluctuates wildly from one tournament to the next. One thing is certain, though:  Kvitova, Ivanovic, Lisicki, and Bouchard are all highly entertaining players to watch. If one of them does produce something special this year, you can be sure it will be worth watching.

So there you have it:  the most significant factor set to wreak havoc on the 2016 WTA season. But who is to say there won’t be many more unseen variables that affect the year in more unexpected ways than anyone could possibly predict? In a sense, that is the beauty of sport. We cannot know for sure what is coming next, and we wouldn’t want it any other way.