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.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.