El Jaish had moved six points clear on Friday with a 2-1 home win over Al Gharafa, but Al Sadd responded in style on Saturday.
Abdelkarim Hassan, Ali Asadalla, Mohammed Kasola, Leandro Montera Da Silva, and Saleh Bader Al Yazidi grabbed the goals in the second half, with the visitors' scant consolation coming from Ebrahima Ibou Sawaneh eight minutes from time.
Third-placed Al Siliyya lost ground as they suffered a dramatic 2-1 home loss to fifth-spotted Qatar SC.
The hosts looked to be heading for three points through Fawzi Ayesh's 10th-minute opener, but Abdulla Al Dyane and Marcinho netted in stoppage time to turn the game on its head.
Finally, early second-half goals from Jesus Meza and Mohammed Razak secured Al Arabi a 2-0 home victory over Al Wakra and fourth-placed Lekhwiya were held to a 0-0 draw at Al Rayyan.