Meghan Markle feared she looked too "eager" after she agreed to a second meeting with Prince Harry the day after their initial blind date in 2016.
According to British biographer Andrew Morton, 65, the now Duke and Duchess of Sussex were "buzzing" after their first encounter, with Harry "enthralled" by the American actress.
Meghan, 37, who at the time was promoting law series Suits in London, was worried she may have made the wrong impression by agreeing to meet the prince again so soon, reports the Daily Mail.
Harry, in turn, was said to be "mesmerised by her beauty and sophistication", after they met in Soho's Dean Street Townhouse, reportedly set up by mutual friend and fashion designer Misha Nonoo.
The revelations come after extracts from Meghan: A Hollywood Princess, written by Morton - who secretly penned a controversial biography for Princess Diana in 1992 - resurfaced online.
Speaking in the biography, Morton writes: "They were mesmerised by one another, Harry enthralled by her beauty, sophistication and perceptiveness.
"She understood him as a man, not a title... As he subsequently confessed, he realised that he would have to up his game.
"Both were buzzing. As she relived that fateful evening in her mind, she perhaps wondered if she had been too eager to accept his invitation to meet again the following day.
"Stay classy, Ms Markle," Morton added to the revealing extract on the newlyweds, who tied the knot in May.
But her fears proved unfounded, and just over a year later the couple announced their engagement in November 2017.
In an extraordinarily candid interview the duo revealed during the interview how they went on just two dates before a besotted Harry invited her on a magical camping holiday in Botswana.
Harry added: "I fell in love with Meghan so incredibly quickly... All the stars were aligned - everything was just perfect. It was this beautiful woman just sort of literally tripped and fell into my life - I fell into her life."
The now Duke and Duchess of Sussex met in the summer of 2016 after being set up on a blind date by a mutual friend, thought to be fashion designer Misha Nonoo.
In November 2016 Harry confirmed their romance after he released a statement asking for his girlfriend's privacy to be respected, revealing she had been subject to a torrent of abuse amid rumours of their romance.
Just weeks later the Prince visited the American actress on set of Toronto-based legal drama Suits, before the couple were first sighted together in London during December 2016.
The duo then made a number of low key appearances together in 2017, before Harry proposed to the stunning Suits star during a cosy night in over a roast dinner in 2017 after a 16-month whirlwind romance.
The couple announced their engagement in November 2017, spending Christmas together with the Queen at Sandringham.
They married on May 19 this year at St George's Chapel at Windsor Castle in England.