Mauricio Pochettino is yet to make a signing this summer, despite the sales of Kyle Walker, Federico Fazio and Clinton N’Jie.
But according to The Sun, Tottenham are set to open their chequebook for the 21-year-old Colombian international.
Spurs have reportedly begun talks in Amsterdam with Ajax over a deal for Sanchez, who won the Dutch club's Player of the Year award last season.
Sanchez has been linked with a move to Barcelona, Chelsea and Crystal Palace, but Spurs look to be most likely to complete a deal for the centre-back.
The Ajax defender admitted in May that he would be interested in a move to Barcelona, and is open to a move away from the Amsterdam Arena.
Sanchez said to Spanish radio station Cadena Ser: “It is a huge compliment [to be linked with Barcelona].
“Who doesn’t want to play for a club as big as Barcelona?
“Everyone wants to be a part of a team like that as they are one of the world’s elite.”
He was part of Atletico Nacional’s Copa Libertadores win in 2016, before moving to Ajax last summer for £4.5m.
Sanchez also played a key part in Ajax’s Europa League campaign last season, where they lost inthe final to Manchester United 2-0.