The Brazilian real was set to record its worst week in over a year on Friday, hit by underwhelming oil auctions and political concerns sparked by a Supreme Court ruling that could lead to the release of former President Luiz Inacio Lula da Silva.

The real dropped about 1% to 4.1398 per dollar, adding to sharp declines this week after major global oil firms snubbed Brazil's biggest-ever pre-salt oil auction, with only the state-owned Petrobras and Chinese state firm CNODC awarded blocs.

Brazil's Bovespa fell 0.9%, but still was on track to the end the week higher, helped by some upbeat earnings reports and optimism over a US-China trade deal.

The Mexican peso held steady, but the Chilean and the Colombian fell amid falling commodity prices.

Adding to the downbeat mood, Brazil's Supreme Court decided on Thursday to end the mandatory imprisonment of convicted criminals after they lose their first appeal, overturning a rule that contributed to the success of Brazil's biggest corruption investigation, the so-called Car Wash operation.

The ruling, which allows convicts to exhaust their appeal options before being locked up, could benefit dozens of high-profile prisoners, among them Lula, jailed last year for taking bribes.

If and when he is released, Lula would be free to engage in politics but would not be eligible under Brazil's Clean Record law to seek elected office for eight years after his first conviction in June 2017.