The Big Four have dominated this generation, but who will own the next?

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Andy Murray, Novak Djokovic, Roger Federer and Rafael Nadal aren't ready to give way to the young players coming up behind them. The Big Four aren't even willing to budge for their 28-year-old contemporary, Juan Martin del Potro, regardless of how happy they say they are to see him making noise on tour again after wrist injuries interrupted his career the year after he won the 2009 US Open -- still his only Slam title. But Milos Raonic, Nick Kyrgios, Dominic Thiem and Alexander Zverev, who are all headed to the Australian Open (Jan. 16-29), are generally given the best chance to lead the next generation of men's tennis champions in 2017, and beyond. They just haven't dominated yet. Raonic is easily the most accomplished of the quartet right now, and Kyrgios is considered the most physically talented. All of which guarantees nothing. Here are some quick takes on what each man needs to do to win his first Grand Slam in 2017, or at least have a personal breakthrough year and climb a little bit closer to cracking the top of the sport.

Milos Raonic, No. 3

The Good: With Federer and Nadal injured much of the year, Raonic trailed only Murray and Djokovic in the year-end world rankings, a career best. The 26-year-old's booming serve is his best weapon, but he also earns raves for his meticulous preparation and professionalism. He's also become better at constructing points once opponents do get his serve back.

The Bad: It's fair to wonder if Raonic will regret splitting with coach Carlos Moya after a year that saw him go 52-17 overall and defeat Federer in their five-set Wimbledon semi with an epic comeback (he lost his first-ever Slam final to Murray in three tight sets). Or does Raonic need someone edgier to challenge him like John McEnroe did as a consultant to Raonic during his Wimbledon run?

The Advice: Raonic said McEnroe urged him to display more outward emotion because the 6-foot-5 Canadian admits he "can get negative quickly." Brad Gilbert, former coach of both Murray and Andre Agassi, says rating Raonic's hopes of winning his first major requires taking a look at his 4-11 career mark against Murray and 0-9 record against Djokovic. "Can he beat one or both of those two guys at the same Slam?" Gilbert asks. "Because he'll probably have to win a major in the coming year."

Nick Kyrgios, No. 13

The Good: Kyrgios is the most physically gifted but most eccentric rising player on tour. The Aussie can cover the court and lash groundstrokes off either side that make the ball sizzle. He makes even the most difficult look easy with his speed and reach. At 6-foot-4, his leaping overhead smashes revive memories of the joy Pete Sampras took in showing off his hops.

The Bad: Kyrgios is a threat to implode at the slightest provocation. In 2016, the 21-year-old feuded with officials of the Australian Olympic team and skipped the Rio Games. He was accused of tanking (not for the first time) at an October match in Shanghai, and fined and suspended for nine weeks after arguing with a fan there, too, who chided him for stunts like walking toward his courtside chair before opponent, Mischa Zverev, had even served out a game point. Those antics were in addition to Kyrgios' usual squabbles with fellow players, linesmen and the media.

The Advice: The ATP ordered Kyrgios to seek professional help. McEnroe ripped him by saying if Kyrgios' heart isn't in tennis, he should find another line of work. ESPN analyst Cliff Drysdale was a little more generous: "He needs to strike a balance between mercurial talent and his baffling temperament. He should not become a subdued automaton but use his fascinating genius. No technical improvement needed."

Dominic Thiem, No. 8

The Good: Thiem has always been in a hurry to get places. Now 23 years old, he turned pro when he was just 18. Unlike Kyrgios, his determination is unquestioned. He played an astonishingly high 28 events in 2016, made his first Slam semi (losing to Djokovic at the French Open) and won four titles, trailing only Murray (eight titles) and Djokovic (seven). Though 6-foot-1 Thiem is three or four inches shorter than the other top young guns, he hits with plenty of power, and he is comfortable on clay, which could evolve into his best surface.

The Bad: Thiem hit the wall by mid-summer and admitted he learned a lesson from overscheduling himself, and then not adjusting. ESPN analyst Pam Shriver also notes Thiem's one-handed backhand makes him a rarity in this era of power tennis and adds she wouldn't be surprised if Thiem develops into a Slam champ later in his 20s, "similar to how Stan Wawrinka did." Gilbert thinks Thiem has the talent to play better defense but must improve his willingness to do so.

The Advice: Drysdale says Thiem could immediately leap forward, but first "he needs only to play the big points better. He does not take enough advantage of his opportunities for a Top 10 player either on his break point chances or when he is break point down on his serve."

Alexander Zverev, No. 24

The Good: The 19-year-old German is young but arcing up. Zverev won just 14 matches in 2015 but improved to an eye-catching 44 victories in 2016, carving out wins over four Top 10 players: Federer, Thiem, Stan Wawrinka and Tomas Berdych. His title win at the St. Petersburg Open made him the youngest champion on tour in eight years, and his advance to finals on clay in Nice and grass in Halle highlights his versatility.

The Bad: Zverev needs to add strength and muscle. Despite his height of 6 feet, 6 inches, his serve isn't yet an overwhelming weapon. At times he seems coltish -- he weighs a scant 189 pounds -- as though he needs to grow into his body. Will he develop the footwork and court coverage it takes to be great? "When you think about the last four champions, the foot speed that Federer, Nadal, Djokovic and Murray all have is off the charts," Shriver says. "I think that needs to be part of the tool box today, too, to be No. 1."

The Advice: Gilbert says, "If Zverev were a stock you'd just put a 'buy' rating on him. The kid's got potentially all the attributes to become a great player. He's well coached. He's professional for his age. He's on the right path. I expect him this year to at least cut his ranking in half and make a run for the semis of a Slam. Then in '18, make the next leap to perhaps winning a Slam."