This has been a historic year in men's tennis, and it ended with yet another surprise at the ATP World Tour Finals last week in London. After a year dominated by Rafael Nadal and Roger Federer, who finished ranked Nos. 1 and 2, respectively, neither appeared in the tour's own championship final. Instead, two players who had never even qualified for the tournament before met for the trophy, with Grigor Dimitrov snatching the win from David Goffin.

You can't be blamed for wondering what on earth can 2018 have in store. Here are five predictions for the coming year:

Grand Slams will produce four different champions

Federer and Nadal divvied up the four major titles in 2017, but that's unlikely to happen again.

Face it, Federer is 36. He has a history of back trouble, which forced him to alter his schedule and pull out of a number of events this year. He'll take good care of himself, limit his appearances and have a few great runs (Wimbledon, anyone?). But the consistency he had in 2017 will be impossible to maintain.

Roger Federer, left, and Rafael Nadal each won two Grand Slams in 2017, a year during which a variety of contenders were plagued by injuries. AP Photo/Andy Wong

Nadal, 31, is in a similar position, but his physical condition is more perilous. He hobbled out of the 2017 season on a bad right knee, the most recent in a career-long struggle with joint problems. Like Federer, Nadal is only in it for Grand Slam titles, at this stage. But he is likely to design his schedule around the French Open.

With the elite ranks back at full strength and legitimate new major contenders emerging, it's hard to see Federer and Nadal repeating their exploits in 2018.

Murray will struggle

Andy Murray is at a critical crossroads. Years of effort and frustration finally bore fruit in his remarkable 2016 season, when he locked down the year-end No.1 ranking in the final match of the ATP year. It had the feel of a career-capping performance.

Also, Murray is a two-time Wimbledon champ and two-time Olympic singles gold medalist. The 30-year-old Scot already is on the short list for being the greatest British athlete of all time. All of which suggests he might be done with the heavy lifting in his career. Another hint: He recently parted ways with the coach who engineered his greatest triumphs, Ivan Lendl.

Murray always has been slow to find his A-game.

"Murray finds his form by playing lots of matches," ESPN's Brad Gilbert, Murray's former coach, told ESPN.com. "It may take him some time to really start clicking."

That's if he starts clicking at all.

Disrupters will threaten the status quo

Goffin, who turns 27 in December, might not have enough weapons to win a Grand Slam in 2018, but he has shown that he can disrupt any draw. He and a number of other players in their mid-20s will make life very difficult for the marquee names, especially if the Grand Slams decide to cut back to 16 seeds (from the present 32).

Dimitrov, Jack Sock, Dominic Thiem, Kei Nishikori, Milos Raonic and others have all been held back by the Big Four and Stan Wawrinka. All of them have improved while patiently awaiting their turn.

Nishikori and Raonic, both Grand Slam finalists, missed a lot of tennis at the end of the year due to injuries. Expect both of them to mount a big push. Thiem, Alexander Zverev, Lucas Pouille and Nick Kyrgios are all 24 or under.

Djokovic will earn back the No. 1 ranking

If the Grand Slam titles are distributed among three or more players, consistency will be the key to becoming No. 1. Novak Djokovic, who's 30, knows what that takes. He might not even have to hit the stratospheric level of play that he tapped into in 2015 in order to fight back to the top.

Djokovic pulled the plug on 2017 during Wimbledon and ended the season at No. 12. He was mired in a slump and suffering from an elbow injury. It was the culmination of a period when he struggled with personal and motivational issues, as well as athletic setbacks. Djokovic will get plenty of help from someone who knows what he has been going through -- his coach and friend Andre Agassi.

Zverev will crack the Grand Slam code

OK, it's ridiculous to criticize a 20-year-old who already is ranked No. 4 in the world and has won two Masters events (Rome and Montreal) with final-round wins over Djokovic and Federer. But Zverev had trouble hitting the high notes at the majors. His fourth-round loss at Wimbledon was his best effort; he lost in the first round at the French Open and won just one match while seeded No. 4 at the US Open.

You can bet Zverev and his team are disappointed and looking at the upcoming year's schedule with different perspectives. His rate of progress suggests that 2018 will be his Grand Slam breakthrough year.