Despite the election setback, Abadi might still be granted a second term in office by parliament and on Monday he called on all political blocs to respect the results and suggested he was willing to work with Sadr to form a government.
With Sadr's Alliance of Revolutionaries for Reform coalition [Saairun] in the lead as 16 of Iraq's 18 provinces are counted, it now appears the Shia cleric will become Iraq's main powerbroker. USA -backed Prime Minister Haider al-Abadi tallied just over 1 million votes and will control 42 seats, and former US puppet Prime Minister Nuri al-Maliki - who turned on the USA and cozied up to Iran - placed fourth and holds about 25 seats. Since he did not run for a seat, he will not be eligible for the role.
Iraqi firebrand political figure Muqtada al-Sadr is set to be announced the surprise victor of the country's elections and prepared for his new status as government titan by making a call for national unity. The Associated Press says he has "in recent years sought to recast himself as a populist, railing against corruption and failing services".
Saturday's election is the first since the defeat of ISIL previous year.
Supporters in his impoverished Baghdad stronghold, Sadr City, were hopeful that victory could spell improvements. "But we have had no results".
Sadr is likely to face fierce opposition from established political forces, who may look to coalesce in a bid to stop him taking control.
His spokesman, Saleh al-Obeidi, said in an interview in Baghdad on Tuesday that Mr. Sadr's movement is seeking allies who agree to its three-plank manifesto - ending the practice of awarding ministries on sectarian quotas, fighting corruption and allowing independent technocrats to manage key government agencies.
While speculation swirls, the next concrete step remains completing the vote count and firming up the final makeup of Iraq's new 329-seat parliament.
Also in the mix is ex-prime minister Nuri al Maliki, a divisive figure blamed for losing territory to Daesh and stirring sectarianism, who has cultivated ties with Iran.