Cosmin Olaroiu's men had previously won seven and drawn two of their top-flight games this term and looked on course to extend their unbeaten run to 10 when Ciel gave them a first-half lead at the Zabeel Stadium on Sunday.
However, Andre Senghor equalised for Al Wasl in the 67th minute, before Maher Jassim grabbed a winner for the hosts two minutes from time.
Second-placed Al Shabab had reduced the deficit at the summit with a routine 2-0 home win over Al Nasr with goals from Edgar Silva and Rashid Hassan on Saturday, meaning they travel to Al Ahli next Friday with the chance to take top spot in the table.
Al Sharjah took full advantage of Al Nasr's defeat to move into third with their 1-0 win over Bani Yas on Sunday, while Al Ain are now fourth courtesy of their 5-2 thrashing of bottom-placed Al Shaab, with former Udinese striker Asamoah Gyan scoring his 12th and 13th goals of the season.
Second-bottom Ajman came from two goals down to earn a point at home to Al Wahda on Saturday, and Emirates slipped to just one place above the relegation zone with a 1-0 defeat at Al Jazira.
In the weekend's other UAE Pro League fixture, Al Dhafra recorded their third win in a row with a 3-2 home success over Dubai on Saturday.
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