The BMW International Open winner was tied with defending Rhys Davies overnight in Morocco, but after a birdie at the par five first he aced the par three second at Golf Du Palais Royal to be three under after two holes.
However, the Englishman did bogey the third to leave Welshman Davies and South Africa's Jaco Van Zyl two behind.
Earlier Denmark's Thomas Björn had shown the leaders what was possible, with a course record 62 to finish six under for the week - propelling the former Ryder Cup star into the top ten.
Should he land his second European Tour title in Agadir, Horsey would not be the first player this season to win with a hole in one in his final round - Spaniard Alvaro Quiros had one en route to landing the Omega Dubai Desert Classic.
Horsey dropped another shot at the long fifth to allow Davies and South African George Coetzee – who made his third blistering start of the week at the Palace with four gains in seven holes – into a share of the lead.
The 24 year old Coetzee and Horsey shared the lead when both birdied the seventh, but the Englishman was back in front when he struck his tee shot at the par three next to six feet and holed.