Former Wimbledon winner Lleyton Hewitt has warned the tennis world not to underestimate Novak Djokovic or Andy Murray as they gear up for their returns.

The duo started the year as the top-two ranked players in the world, however, poor performances along with injuries saw them drop in the rankings, with both players last featuring at SW19 in July.

Having emulated Roger Federer and Rafael Nadal by taking time off until the new year, Djokovic and Murray are set to return to action, with the pair both confirmed to feature at the 2018 Australian Open.

There are doubts, however, on whether they can return to their previous levels that saw them dominate the entirety of 2016, especially with Federer and Nadal enjoying career renaissances in 2017.

Hewitt shares these doubts as well but claims he would never write them off, especially for the Australian Open next month.

"I don't know if they'll be as dominant as they were," Hewitt said, as quoted on SBS. "For them to pick up where they left off a couple of years ago is not that easy.

"But I would never write those guys off. They've played so well in Australia in hot conditions and also in night matches on Rod Laver Arena. If they can get through that first week and give themselves a chance, then they'll be tough to beat."

With the returns of many other injured players like Stan Wawrinka, Milos Raonic and Kei Nishikori, 2018 is heating up for a great start.

However, because these players are coming back from injury, Hewitt believes it is the perfect time for the younger Australian players like Nick Kyrgios, Jordan Thompson and Thanasi Kokkinakis to make a mark at Melbourne Park.

"It's an interesting one," Hewitt added. "You've got (Stan) Wawrinka, (Milos) Raonic, (Kei) Nishikori - a lot of top guys coming off an extended break from injuries or surgeries at the end of the year.

"So the door is open for some of these younger Aussie boys."