Nadal finished the season as world number one, but Federer beat him in three finals, including one Grand Slam final.
The two greats shared the majors between them last season and Nadal leads his rival in head-to-head clashes – 23-15.
The Spaniard cut his season short due to injury, but he is targeting a return to action at the Abu Dhabi exhibition event at the end of this month ahead of the Australian Open which gets underway in January.
When pressed on whether Federer and Nadal can stay at the summit next year, Hewitt said: “You wouldn’t think so but I don’t know if those two are normal, especially Roger.
“At his age, he’s wound back the clock.
“Even against Rafa, he’s done the best he ever has against him.
“I wouldn’t say there’s anything those two can’t do.”
Hewitt suggested earlier this week that Federer is the favourite to win the Aussie Open.
He said: “Federer has been so dominant, especially in big tournaments.
“He’s gonna go in – even at the age of 36 – as probably the favourite.
“Especially the way Nadal obviously had to pull out of London.
“But there’s going to some opportunities for some young guys to stand up and maybe win a grand slam.”