Both Djokovic and Murray were sidelined with injuries for the majority of the 2017 season with Rafael Nadal and Roger Federer benefitting from their absence.
Murray was the world No1 going into the campaign but a persistent hip injury has meant he hasn’t played competitively since his Wimbledon exit.
And Djokovic succumbed to an ongoing right elbow that ended his season midway through the year.
The two tennis stars are expected to return to action in the New Year at the Australian Open.
But Hewitt, while admitting the duo should never be ruled out, doubts whether they will dominate like they did before.
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“I don’t know if they’ll be as dominant as they were,” Hewitt told Sky Sports.
“For them just to pick up from where they left off a couple of years ago, it may not be that easy. But don’t write those guys off.
“Both those guys, Djokovic and Murray, have played so well in Australia. They play so well in the hot conditions and also in the night matches on Rod Laver Arena.
“If they can get through that first week and give themselves a chance then they’re going to be tough to beat.
“But it’s an interesting one. 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.”