The Spaniard defeated South Africa's Kevin Anderson in the US Open final to win his fifth title and second Grand Slam of the year on Sunday (10 September).
As a result of his overall performances this year, Nadal became world number one again for the first time since 2014 as well.
Shiras, now a tennis analyst, believes that while the 31-year-old got a favourable draw in Flushing Meadows, he took full advantage of it and is arguably playing the best tennis of his career right now.
"Nadal is playing maybe the best of his life – you could almost say that," Shiras said, as quoted by Sky Sports.
"He has benefited historically from one of his most favourable draws at the US Open. I don't think Rafa normally gets draws this favourable but he took full advantage of it.
"We all knew Nadal was so effective at the net but some people are whispering to me that he could be an even better volleyer than Roger Federer.
"That's a debate for another time, but the fact that he's doing so well when he comes forward shows signs that if he can end points earlier and be tactically-layered like he is in terms of his defensive play, the sky's the limit for Rafa."
While Nadal insists he is not focused on overtaking long-time rival Federer's tally of 19 Grand Slam titles, Shiras believes that the 16-time Grand Slam winner could add at least four or five more to his haul, given how physically fit he is at the moment.
"The way he looks right now, Rafa could add at least four or five more Grand Slam titles," Shiras said. "So he has a very good chance to catch and surpass Roger just because of how he looks physically."