The Reds were reportedly keen on the 18-year-old last summer but their approach was turned down by Mendes' current club Sporting Lisbon.
Mendes has since established himself in Sporting's starting XI this season, playing his part in the side moving nine points clear at the top of the table.
He was rewarded for his performances with a new contract in December. As part of that deal his new release clause has been bumped up from €45m to €70m.
Despite this price increase, Liverpool are still said to be tracking the player's development, and could be tempted to make an offer in the next transfer window.
They aren't Mendes' only admirers with O Jogo also mentioning Juventus, AC Milan, Inter Milan and Real Madrid as fellow interested parties.
GIVEMESPORT's Sam Brookes says...
Liverpool attempted to strengthen their options at left-back last summer by signing Konstantinos Tsimikas from Olympiacos.
However, thigh and knee problems have limited him to just five appearances in all competitions this term. When the Greek international has featured, he has looked some way below the standard of the side's first-choice full-back Andy Robertson.
With that in mind, it is no surprise that Liverpool are looking to bring in another fresh face in this position.
Paying €70m (£60.4m) for a back-up full-back is far from ideal, so Liverpool are likely to try to bring Sporting's asking price down.
Mendes is only 18, though, and would be able to learn from one of the best in the business in Robertson. Having played regularly this season, he will have gained valuable experience, and could allow Jurgen Klopp to rest Robertson at vital moments next year.
After this season's horrific injury crisis, the need for viable rotation options going forward is more prevalent than ever at Anfield.News Now - Sport News