Group leaders Brazil face Paraguay in Sao Paulo and will move on to 33 points if they win what would be their eighth consecutive qualifier.
Ecuador and Chile are tied in fifth place with 20 points, behind Uruguay on 23, Argentina 22 and Colombia 21.
If Ecuador do not beat Colombia and Chile fail to take maximum points from Venezuela then they would not both be able to overtake Brazil.
The top four teams in the South America region qualify automatically for Russia 2018 and the fifth-placed side go into a playoff against a side from Oceania.
There will be four more rounds of games after Tuesday's matches.
Brazil are the only team to play in every World Cup but they were humiliated on home soil in 2014 when eventual winners Germany hammered them 7-1 in the semi-finals.
Coach Tite has got them back on track after Dunga's unhappy spell in charge and their recent form - a South American record seven qualifying wins in a row - has established them as one of the favourites to win the World Cup for a record-extending sixth time.
They are strongly fancied to beat seventh-placed Paraguay, even though the visitors have won or drawn their last four competitive fixtures in Brazil.
Thirty two teams will take part next year's tournament, with hosts Russia already assured of a place.
(Writing by Andrew Downie, editing by Ed Osmond)
