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Markets had expected former president Pinera to easily be elected to succeed outgoing leftist President Michelle Bachelet.
But he secured fewer votes than forecast in Sunday's first-round election and now faces a tougher-than-forecast battle against center-left Alejandro Guillier ahead of a Dec. 17 runoff.
Pinera has promised to cut the corporate tax rate and slash red tape in the world's top copper producer, while Guillier wants to deepen Bachelet's progressive policies.
Neither candidate would mark a departure from Chile's longstanding free-market economy.