The 18-year-old is the first British woman to win a grand slam singles title since Wade more than 40 years ago.
That she did it as an 18-year-old qualifier ranked 150 and without dropping a set made it an achievement hard to comprehend.
From her home on Long Island, Wade told the PA news agency: “When you think about what she’s done, to be a qualifier and win all those matches in a row and being so young, it’s just exceptional.
“It’s more than exceptional. It’s something that when she’s out of her tennis career she can look back and think, ‘Oh my goodness, was that really possible?’ It really is an impossible success that she’s had.”
Wade watched from the stands at Flushing Meadows as Raducanu made fellow teenager Leylah Fernandez her final victim, defeating the 19-year-old Canadian 6-4 6-3.
Raducanu has been both calm yet wide-eyed with delight at her achievements, and spoke again about her pride at having Wade there to see her triumph as the three-time grand slam singles champion beamed and applauded.
She had no hesitation at all about handing over the mantle, saying: “I’ve always said the fact that I won the US Open, that was quite a feather in my cap, but it wasn’t that I was the last British winner that was making me proud.”
Wade has remained an avid follower of tennis since her playing days and sees in Raducanu the same star quality she did in the likes of Steffi Graf Martina Navratilova and Serena Williams.
The teenager’s life will undoubtedly change on and off the court but Wade hopes she can adjust and go on to become a serial winner, too.
“In tennis or in anything you can’t expect there to be a real, legitimate star every year or every two years,” said Wade.
“When you think of the young stars that there have been all the way back to Maureen Connolly, their reputations went ahead of them – if it was Chris Evert or Tracy Austin, and certainly Martina and Steffi.
“Steffi was brilliant at 18. She just came on the scene and you knew she was going to just keep going forward winning everything. And of course Serena.
“You get the same sort of feeling with Emma that she’s just better than her contemporaries and better than probably a lot of her seniors and she’s the exception who’s going to go forward.
“We know that there’s going to be a lot to contend with and especially in the next year. Wimbledon I think is much harder for an English person anyway. Expectation is going to be quite weighty.
“It’s going to be a massive change for her. She had a little feel of that at Wimbledon suddenly being very much in the public eye and people knowing her but now she’s on every single front page and she got a message from The Queen.
“She’ll get offers of endorsements left, right and centre and she’s just got to have some real guidance with discerning people around her to help her. But I think she’s a cut above intellectually as well.
“She handles herself with tremendous poise and, as long as she keeps her feet on the ground and is selective with what she does. I always think the most important thing when you’ve done unbelievably well is to make sure you understand you go up one step and you have to consolidate that.
“When you have, then you go up another step. She’s got the big prize very early so she has to keep setting goals that motivate her and keep enjoying playing.”
A photo tweeted by the tournament showed Wade and Raducanu sharing an embrace backstage, and Wade said: “That was so cool.
“Kim Clijsters was up in the President’s Box as well. I said, ‘Let’s go down and see if we can see her as she comes off the court’. The tournament director Stacey Allaster said, ‘No, no you can’t go down there’. We said, ‘We’re going down there’.
“After about 10 minutes she came off the court with all the (security) guards and I said, ‘Emma, can we have a hug please?’ She said, ‘Of course’, so we were all lining up for hugs. She would have hugged the whole stadium I think. It was lovely.”
Raducanu is not yet old enough to drink champagne in the United States and she was planning to celebrate her success with some chocolate frozen yoghurt.
The letter from The Queen would have given the teenager some idea of the huge impact her success has had back home, with sporting stars and celebrities falling over themselves to congratulate her.
She told the BBC: “It meant everything to get a message from Her Majesty. She’s such a great inspiration and role model for the whole country so, to have a note from her, I was extremely honoured and very, very grateful that she took notice of my tennis.
“I can’t believe it. I’m maybe going to frame that letter or something.”
Channel 4’s late deal with Amazon Prime Video to show the match live on free-to-air TV paid off handsomely, meanwhile, with a peak of more than nine million viewers tuning in.