Injuries played a widespread role throughout 2018 but the Big 3 remained atop the rankings nevertheless. While Novak Djokovic seems as dominant as ever, a few young upstarts are hoping to join Alexander Zverev, Dominic Thiem and Kei Nishikori as the only non-30-year-olds inside the top 10.
No. 1 – Novak Djokovic: Djokovic's close to 2018 was sensational as he wrapped up the No. 1 ranking and took home titles at both Wimbledon and the US Open. The 31-year-old Serbian did have a slight hiccup at the end of the year, though, as he lost two straight finals in Paris and London to Karen Khachanov and Zverev. Djokovic will open his season in Doha, and he'll be the solid favorite at the Australian Open, where he'll be looking for his sixth title and 15th major title overall.
No. 2 – Rafael Nadal: An abdomen injury forced Nadal to end his 2018 campaign early as he retired in the US Open semifinals against Juan Martin del Potro. The 32-year-old Spaniard will have had close to four months to heal by the time he begins the 2019 season during its opening week in Brisbane. Nadal has bounced back from injuries numerous times in his career, so there's no reason to believe he won't do the same in the upcoming season. In 2018, Rafa was forced to retire in the quarterfinals of the Australian Open last season, but he went on to win his 11th French Open title.
No. 3 – Roger Federer: After he takes part in the Hopman Cup exhibition to open the year, the two-time defending Australian Open champion will officially open his 2019 campaign at the year's opening major as he continues his trend of skipping smaller warmup events. Considering the 37-year-old has won the year's first Grand Slam in consecutive seasons, he'll be regarded as the second favorite at the event, only behind Djokovic, who has won five Australian Open titles overall along with the last two majors.
No. 4 – Alexander Zverev: The 21-year-old German will be looking to build off his strong end to the 2018 season, in which he took home the year-end title in London. Zverev's only warmup for the 2019 Australian Open will come at the Hopman Cup exhibition, where he'll be teaming up with Angelique Kerber for Germany. Zverev has yet to advance past the quarterfinals of a major, and last season in Australia he disappointingly fell in the third round. All eyes will be on him during Grand Slam events this year.
No. 5 – Juan Martin del Potro: The 30-year-old Argentine is set to return from a knee injury at the Australian Open, and he'll also get an exhibition event in Kooyong to test out the knee before the official competition begins. Del Potro has a long injury history, and he tends to be cautious in his return from injury, so it's not out of the question that he will pull out of year's first major if he isn't feeling 100 percent. Del Potro had one of the best seasons of his career in 2018 before the injury, and he'll be looking to regain that form in 2019. In 2018, del Potro disappointed at the Australian Open with a straight-set loss in the third round.
No. 6 – Kevin Anderson: The 32-year-old South African had the best season of his career in 2018, so he should enter into 2019 with more confidence than ever. He'll open his 2019 campaign in Pune, where he'll be the top seed and the favorite to claim the title. Anderson suffered a first-round loss at the Australian Open last season, and he'll be looking to shake off those bad memories at the year's first major.
No. 7 – Marin Cilic: Cilic's 2018 campaign was pretty strong, as he went 44-20, but he disappointingly captured just one title. This season, he was supposed to open play in Prune but pulled out just days before with lingering knee pain. The 30-year-old Croatian blitzed all the way to the final of the Australian Open last season, where he tested Roger Federer in a five-set battle. Cilic will have pressure this season at the Australian Open, because if he doesn't fare well, he will see a large chunk of ranking points fall off. That is, if he's even able to play at all.
No. 8 – Dominic Thiem: The 25-year-old Austrian had his best-ever result at a hard-court major at the end of last season when he reached the quarters of the US Open. That should give him confidence on hard courts in 2019, as he'll get things started in Doha, where he'll be the No. 2 seed behind Djokovic. Thiem did better than expected at the Australian Open last season, where he was ousted in the fourth round in a five-set battle with Tennys Sandgren.
No. 9 – Kei Nishikori: Nishikori will kick things off in 2019 at the Brisbane International after his strong finish to the 2018 season which saw him catapult back into the top 10. The 28-year-old from Japan went 43-21 last season, but also finished 0-3 in finals appearances. Nishikori will be looking for a push back into the top 5 this season, and that's quite possible, if he stays healthy.
No. 10 – John Isner: The 33-year-old American had the best season of his career in 2018, as he captured two titles and finished in the top 10. This year, he'll kick things off in Auckland, where he will be the top seed. The year's opening major hasn't been kind to Isner lately, as he lost in the opening round of the Australian Open last season, and fell in the second round in 2017.
No. 11 – Karen Khachanov: The 22-year-old Russian will be on everyone's radar this season after he closed out last year with an emphatic title in Paris over Djokovic. He'll open up his 2019 campaign in Doha, where he will be the No. 3 seed. Khachanov is ranked just outside of the top 10 at the moment, but he looks poised to enter that club really soon. He could make a deep run at the Australian Open if his draw is favorable.
No. 15 – Stefanos Tsitsipas: The 20-year-old from Greece made a big leap last season, from being ranked No. 91 at the start of the year all the way up to No. 15. Tsitsipas is opening his 2019 season in Sydney as he looks to continue his climb up the rankings. He'll be one to watch this season, as a push into the top 10 is certainly plausible.
No. 16 – Daniil Medvedev: The 22-year-old Russian made a name for himself to close out 2018, as he racked up two titles in the second half of the season in addition to his title in Sydney to start the year. The success brought him all the way up to No. 16 in the world, and he'll look to build on that as he opens his 2019 season in Brisbane. Medvedev has an unorthodox game to go along with confidence in himself. A push into the top 10 would be slightly surprising, but it's not out of the question.
No. 27 – Denis Shapovalov: The 19-year-old Canadian had an up and down 2018 campaign as he finished with a 35-28 record. Many expected quicker success for Shapovalov, who possesses some serious shotmaking ability. In 2019, he'll kick off his season in Auckland. In order to climb higher in the rankings, he'll need to show more consistency than he has so far in his young career.
No. 31 – Alex de Minaur: The fiery 19-year-old Australian will be interesting to watch in 2019, after he vaulted all the way from No. 208 in the world at the start of last year to No. 31. De Minaur will get things started in 2019 on home soil in Brisbane, where he reached the semifinals last year. Considering de Minaur's competitive attitude and consistent game, it would be a surprise if he didn't advance into the top 20 this year.
No. 66 – Stan Wawrinka: Wawrinka slowly but surely found some form in the second half of 2018 while making a return from injury, as he posted some good wins to go along with some bad losses. The 33-year-old from Switzerland will open this year in Doha, where it's unclear if he may have to go through qualifying. Expect the three-time Grand Slam winner's ranking to jump into at least the top 20 sooner rather than later.
No. 257 – Andy Murray: After a short-lived 7-5 return to the Tour in 2018 after a hip injury, Murray will look to regain his old form in 2019 as he begins play in Brisbane. Murray's ranking is all the way down at No. 257 at the moment, and his play was spotty in 2018 as he looked to improve his ranking. The 31-year-old Brit's form is one of the biggest question marks on the men's tour in 2019, but considering his past success, the wise bet would be that he'll find his form once again.