Al Shamrani struck in stoppage time to cancel out Victor Simoes' 12th-minute opener and extend his team's unbeaten run to 13 games and move them two points ahead of second-placed Al Hilal.
Elsewhere, Al-Ittihad climbed to fifth spot with a 7-0 hammering of Hajer.
The hosts were four goals to the good at half-time thanks to Osama Al Harbi, an Abdallatif Al Bahdari own goal, Saleh Al Saqri and Mohammed Noor, and further strikes after the interval from Mohammed Al Rashed, Noor and Redha Tukar sealed the rout.
Mohammed Al Sahlawi looked to have nicked a point for Al-Nasr when his 71st-minute goal got his side back on level terms at Al-Qadissiyyah following William Tiero's early opener, but Talal Al Shmaly bagged the winner for the hosts two minutes later.
Finally, Yahya Al Muslem and Waleed Al Jezany were on target in the final 15 minutes to earn Al-Raed a 2-0 triumph over second-bottom Al-Tawon.