Talks for Porro have dragged on for most of the month, with the clubs set to resume negotiations on Thursday.
Having had at least one bid rejected, Spurs have indicated a willingness to meet Porro's €45million (£39m) release clause and believe Sporting are now dragging out negotiations. The structure of payments is still thought to be on the table, however.
The Portuguese side face rivals Porto in the Portuguese League Cup final on Saturday which may be a factor in their determination not to lose Porro before the weekend.
Sporting are also keen to line up a replacement this month and have made an approach for Brighton's Tariq Lamptey on loan.
Spurs had offered players to Sporting in part-exchange for Porro and tried to reduce their 40 per cent sell-on fee for Marcus Edwards, their former academy graduate who now plays for the Primera Liga club, as part of a deal.
Despite the back and forth, there is now confidence that the 23-year-old Spaniard will become Spurs' second January signing following the capture of Villarreal winger Arnaut Danjuma on loan yesterday.
Head coach Antonio Conte has long wanted an upgrade at right wing-back, where Matt Doherty and Emerson Royal have both underwhelmed in the role.