Leonardo Jardim's side were held at home by Nacional in their final game before the winter break and dropped points again on Saturday against the league's fourth-placed team.
Sporting are now a point clear of Porto and Benfica, who lock horns on Sunday with both sides knowing a victory would secure top spot.
Estoril still have only two home wins to their name this season and are eight points behind third-placed Benfica.
Saturday's other Primeira Liga game also ended in a draw after substitute Joeano salvaged a 1-1 draw for Rio Ave at home to Maritimo.
Heldon's 87th-minute strike appeared to have secured all three points for the visitors, but striker Joeano struck two minutes later to deny Pedro Martins' side.
The home side then had Tarantini sent off for a second bookable offence in a dramatic finale.
Maritimo would have gone above their opponents with a victory, but instead occupy 10th spot with Rio Ave up to eighth.
‘Maybe I’ll be signed by Wrexham, who knows? English football would suit me – if an offer came, I wouldn’t think twice, I’d go immediately’ Euro 2024 cult hero reveals his UK dream
‘Managing Leeds? It was an option that appeared, but it wasn’t the right timing. I decided it wasn’t a good idea to leave the club I was at mid-season’: Premier League boss admits to turning down opportunity to replace Jesse Marsch in 2023
