* No candidate secured commanding win in first round
* Old political arch-rivals back opposing candidates
* Smooth vote needed to draw back investors
* Madagascar's economy shrunk 4 pct after 2009 coup
By Alain Ilioniania
ANTANANARIVO, Dec 18 (Reuters) - Madagascar stages a run-off presidential election on Friday, but old rifts may persist, extending a crisis begun by a coup five years ago that deterred investors and donors of aid to one of Africa's poorest nations.
Neither candidate scored a commanding victory in October's first round. Both rely on supporters of their respective sponsors, outgoing President Andry Rajoelina and the man he deposed with the army's help in 2009, Marc Ravalomanana.
Voters may not deliver a clear mandate to either Jean Louis Robinson, an ally of Ravalomanana, or Hery Rajaonarimampianina, a former finance minister backed by Rajoelina.
Parliamentary polls also taking place on Friday could lead to one camp holding the presidency and the other controlling the legislature, perhaps forcing them into a power-sharing deal.
Such a compromise might please donors and many Malagasys, exhausted by five years of political stalemate, but some fear rival leaders will instead prolong a paralysing confrontation in which the economy has shrunk 4 percent since the coup in 2009.
"Elections should end the fuzzy situation of transition and return the constitutional order and international recognition," said Didier Andriamanantena, a 22-year-old student, before the vote, which will be followed by a count likely to last days.
"But they are not the solution to all our problems. The new leaders should quickly face a difficult economic and social situation," he said, speaking on the streets of the capital where a cash-strapped government has let rubbish pile up.
Smooth elections could help restore the confidence of mining and other investors, revive the battered tourist industry and re-open the aid taps to a country of 22 million people, of whom nine out of 10 live on less than $2 a day.
But much hangs on how the loser reacts and whether the army, which had backed Rajoelina, stays in its barracks this time.
DEALING WITH THE ARMY
In the first round Robinson secured 21 percent of the vote, while Rajaonarimampianina won 16 percent, both far short of the 50 percent plus needed for outright victory.
"Unless Robinson makes a deal with the army to guarantee that their concerns are addressed, I do not see massive support from the army for him," said Lydie Boka, manager of consultancy Strategico. "There is no way it can and will remain neutral."
The economy of the world's fourth largest island, which boasts unique flora and fauna as well as rich mineral deposits, had grown strongly at around 7 percent a year before the coup.
Multinationals had then scrambled to develop or explore nickel, titanium, cobalt, iron, coal and uranium deposits, as well to hunt for hydrocarbons - the island's maritime boundaries border Mozambique where plentiful gas has been found.
Interest slowed sharply after 2009. Issuing new mining licences stopped. Now, low metals prices mean Madagascar will have to work harder to draw investors even if order returns.
"The free, transparent and credible election of a government in Madagascar, and the resumption of international aid following the recognition of that elected government, would be positive for investment in the country," mining investor Sherritt International said in a statement to Reuters.
The Toronto-listed firm owns 40 percent of the $5.3 billion Ambatovy nickel mine, which started production last year.
The other major mining operator is London-listed Rio Tinto , holding 80 percent of a mine producing ilmenite, an ingredient used in paints, paper and plastics.
Other mining investors have scaled back plans or left.
"I hope this election puts an end to several years of crisis," said Jeannette Ravonisoa, a 40-year-old vegetable seller. "We suffer too much from the current situation." (Additional reporting by Richard Lough in Nairobi; Writing by Edmund Blair; Editing by Alistair Lyon)