UPDATE 5-Argentina's Fernandez nearly tied in Senate primary

(Recasts with vote nearly tied)

BUENOS AIRES, Aug 13 (Reuters) - Argentina's former populist leader Cristina Fernandez and business-friendly President Mauricio Macri's preferred candidate were nearly tied in a closely watched Senate primary on Sunday, with few votes left to be counted.

With 87.97 percent of polling stations counted, Macri's coalition led by former education minister Esteban Bullrich had 34.31 percent of votes compared with 33.98 percent for Fernandez's list in Buenos Aires province.

Bullrich's lead narrowed significantly late in the count in a hotly contested battle to represent the province that is home to nearly 40 percent of Argentina's electorate.

Fernandez had been expected to win by several percentage points according to the final polls last week, causing investors to fear a comeback in Congress could pave the way to her running for president in 2019 and ending Macri's reform agenda.

The compulsory primary vote essentially serves as a detailed poll ahead of the Oct. 22 election for one third of the Senate and half the lower house of Congress, because no major candidates are being challenged from within their own parties. (Additional reporting by Nicolas Misculin, Walter Bianchi, Luc Cohen and Maximiliano Rizzi; Writing by Caroline Stauffer and Luc Cohen; Editing by Andrew Hay, Chris Reese and Paul Tait)