Can Novak stay at the top of the rankings throughout 2021? Will Nadal be able to remain a threat in best-of-5 tennis outside of Roland Garros? Who'll take the next step towards tennis super-stardom? 

While it may be moving slowly, the changing of the guard at the top of the men’s game is occurring right before our eyes, and much like we're seeing within the WTA, there's plenty of cause for optimism regarding a new crop of elite players whom are beginning to make their presence felt.

Here are five up and coming men on the ATP tour to watch out for ahead of both the Australian Open, and throughout the 2021 season. All five will be around at the top level for years to come, so it's well worth getting to know a little more about them now.

RELATED: 5 Outsiders Who Could Win The 2021 Australian Open Men's Singles Title

Jannik Sinner

Age: 19

Rank: 36

He may fall into a seeding with another withdrawal; however, the Italian is well and truly a player to watch at the Australian Open and throughout the 2020s. Sinner put together some impressive performances at the lower levels throughout 2019, then backed it up with a run to the semi-final of the ATP tournament in Antwerp, before winning the Ortisei Challenger. Although 2020 started a little slowly for the 19-year old, he returned with a vengeance late in the season. Cramps cruelled him of a first round upset of Karen Khachanov at the US Open, before reaching the Roland Garros quarter-final where he lost to Nadal. Sinner then claimed the ATP title in Sofia, defeating Alex De Minaur along the way. 

Sinner has an incredibly high ceiling, and there is likely to be patches of form in the coming weeks that will reflect this.

Great 2 weeks in Adelaide! What an experience. Thanks @tennisaustralia.

So grateful to @rafaelnadal and his team for having me practice with them. Amazing to spend time learning from one of the greats. Learnt some cheeky football skills too, thanks Team Rafa! 💪🏼👊🏼 pic.twitter.com/UlI0DGe8La

— Jannik Sinner (@janniksin) January 28, 2021

Carlos Alcaraz

Age: 17

Rank: 138

It must be tough being compared to Rafael Nadal as a teenager on tour, however that's what Carlos Alcaraz is currently dealing with at 17 years of age. It only takes a quick look at his style and ability on clay for his age to realise the incredible potential in his game, with lofty expectations on his shoulders, just like Sinner. Whilst clay is his preferred surface, this will be the first main draw at a grand slam for Alcaraz, having won his way through qualifying in Doha a couple of weeks ago. It will be interesting to see how he handles the longer best-of-5 set format, however he's one to keep an eye on as the tour progresses towards the clay season. Remember the name. 

Trabajo de coordinacion, seguimos mas fuertes que nunca!!#AO
Coordination work, we continue stronger than ever !! # pic.twitter.com/eGOhOuc4a5

— Carlos Alcaraz (@alcarazcarlos03) January 25, 2021

Alexander Bublik

Age: 23

Rank: 45

After being exposed to the top level of the tour a little early with some grand slam wins in 2017, Alexander Bublik became a little lost in the system. Struggling with form and motivation, particularly at the lower levels, there were question marks as to whether he would ever reach his potential. If the start to 2021 is any indication, there may be a spike in form from the Kazakh this year. He kicked off the year by reaching the final of the tournament in Antalya, where he had to retire early in the first set against Alex De Minaur. With wins over Berrettini and Chardy en route to the final, it's solid form that has allowed him to reach a career high ranking of 45. Cut from the same cloth as Nick Kyrgios in terms of on-court style and entertainment, his career success will ultimately come down to how badly he wants to be successful. The talent is there, and it looks as though the week-to-week application is now starting to follow. Perhaps the COVID tour suspension period helped Bublik and others to re-assess their priorities?

Is Alexander Bublik the 🤴 of trick shots?

The young 🇰🇿 takes out No. 8 Gael Monfils in four sets at #RolandGarros pic.twitter.com/QgWNztdKPh

— US Open Tennis (@usopen) September 28, 2020

Lloyd Harris

Age: 23

Rank: 90

There had been questions regarding who Kevin Anderson would pass the torch to for South African tennis moving into the next decade. It looks to have been answered with Lloyd Harris. As a taller player more reliant on building his serve and power game a little more, things have not come easily for Harris. He has progressively improved his ranking year-on-year since 2015, and is now at the point of trying to capitalise on ATP main-draw appearances. He surprised many by reaching the final of the ATP even in Adelaide last January, defeating the likes of Garin, Djere, Carreno Busta and Paul on his way to falling to Andrey Rublev. He also managed to give Roger Federer a scare in their meeting at Wimbledon in 2019 by taking the first set in their first-round match.

Harris went 25-15 over the course of 2020 and started to look as though he finally felt at home at the top level. It'll be interesting to see how much improvement is made in his ranking now he is inside the Top-100.

Christopher O’Connell

Age: 26

Rank: 121

Whilst he doesn't fit the mould of the other younger players on the list, the Australian still fits the description of a player looking to break through to the next level. Not long removed from washing boats and collecting Centrelink cheques, O’Connell was the breakout star at the lower level through 2019, ending with an 82-25 record, going from unranked to 128th in the world. The COVID tour suspension came at an awful time for O’Connell, cruelling him of the significant momentum he'd gained over the previous 15 months. He looked to find his form late in 2020 before rolling his ankle on match point up 7-6 5-2 at Challenger level, causing the Australian to retire. 

He has had 3 months to recover, and was the recipient of an Australian Open wildcard, so it will be interesting to see if he can make an impact. Hopefully he finds himself in a better part of the draw throughout the season, unlike in 2020, where he ran into Andrey Rublev in the first round and Daniil Medvedev in the second round at both the Australian and US open.

Christopher O'Connell just bageled Andrey Rublev?

Christopher O'Connell just bageled Andrey Rublev. #AusOpen

— Ben Rothenberg (@BenRothenberg) January 21, 2020

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