Hugo was unmarked when he headed home a cross from fellow midfielder Junior Cesar, who broke from deep in defence in a swift break down the right, to give Sao Paulo their sixth win in a row, lifting the title holders to fourth in the standings.
Washington put Sao Paulo ahead in the 25th minute from fellow striker Borges's headed pass.
Sport equalised five minutes from time through Fabiano after Sao Paulo defenders Miranda and Renato Silva, and Sport midfielder Wilson, were sent off between the 70th and 75th minutes for second bookings.
"It was a match played by great champions ... It was a heroic victory because we had two players sent off," Washington told reporters.
Leaders Palmeiras had their advantage cut to two points after being held 1-1 at home by Botafogo, while second-placed Goias beat Vitoria 3-2.
At the halfway point in the championship, Palmeiras have 37 points and Goias 35. Sao Paulo and Internacional, who have two matches in hand, are on 33.
Atletico Mineiro dropped out of the top four after a 2-0 defeat away to Corinthians, who notched up their first win in six matches since losing former FIFA World Player of the Year Ronaldo with a broken hand.
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