"These elections are very important for Guinea-Bissau because they will enable the consolidation of democracy and credibility ... and enable stability and development," da Costa said.
Sanha, 62, was interim president for a year following a 1998-1999 civil war, while Yala was elected president in 2000, but overthrown in a bloodless coup three years later.
The vote was triggered by the killing of Joao Bernardo Vieira, Guinea-Bissau's long-time president, by soldiers on March 2, in an apparent revenge attack following the assassination of army chief General Batista Tagme Na Waie in a bomb attack.
In June, the army killed two senior political figures in what they claimed was an operation to foil a coup plot.
The murder of Vieira, who ruled Guinea-Bissau for much of the past 25 years, came about a decade after the military ousted him during a previous term as president.
The run-off round was originally to have been held on August 2 but was brought forward to encourage a higher turnout as the later date could have interfered with harvest work in the predominantly rural country.
Since gaining independence from Portugal 35 years ago, Guinea-Bissau has experienced numerous coups, countercoups and a civil war.