It comes after the 18-year-old from Kent faced harsh criticism over her withdrawal from Wimbledon this summer, during which she retired early from her clash with Australian tennis player Ajla Tomljanovic in the fourth-round due to breathing difficulties.
Her withdrawal from the competition sparked disapproving comments, including by American former tennis professional John McEnroe, who claimed Ms Raducanu dropped out because she was unable to handle the occasion.
Piers Morgan took to Twitter to say the teenager needed to “toughen up”.
In his latest tweet following the Sunday-night championship game, Morgan referred to Ms Raducanu as the “Wimbledon choker” and also suggested her victory in the US was a result of her taking his advice.
During a visit to a British Gas training academy in Leicester on Monday, the Prime Minister told reporters: “I think it was stunning and I am sure, like everybody else, I watched the whole thing with my heart in my mouth.
“I couldn’t believe how hard they were walloping that ball.
“An incredible sort of resilience under pressure, that was what came through to me.
“There were so many moments where you thought the match was going to go the wrong way and then she held it together – it’s just fantastic. So, many, many congratulations.”
Ms Raducanu is the first-ever qualifier to win the tournament, taking the trophy in New York’s Arthur Ashe Stadium after defeating 19-year-old Leylah Fernandex without dropping a set.
Her rapid rise to sporting stardom has seen her go from number 338 in the world to number 23. She is now the new British number one.
Raducanu has meanwhile said she will frame her letter of congratulations from the Queen following her US Open win.
A personal message to Raducanu from the Queen, which was posted on the Royal Family website, read: “It is a remarkable achievement at such a young age, and is testament to your hard work and dedication.”
Raducanu, in an appearance on US breakfast show Good Morning America on Monday, said: “I was so honoured and grateful to receive a note from Her Majesty. It meant everything to me and I will probably frame that letter.”