Boca Juniors beat River Plate 2-0 to win the Argentine ''superclasico'' on Sunday, dealing a double blow to River's faltering league campaign.
Meanwhile, league leader Velez Sarsfield lost 3-2 at Lanus and third-placed Olimpo suffered a 2-1 home defeat against Independiente to give encouragement to the chasing pack.
River goalkeeper Juan Carrizo punched into his own net from Pablo Mouche's corner to give Boca the lead in the 28th minute and Martin Palermo headed the second from close range three minutes later.
Palermo, Boca's record goalscorer, will retire at the end of the season and the Bombonera crowd gave the teary-eyed 37-year-old a prolonged standing ovation when he was substituted in the 72nd minute.
Boca's Clemente Rodriguez and River's captain Matias Almeyda were sent off after clashing in stoppage time, with a visibly furious Almeyda needing a police escort off the pitch.
The win moved Boca within a point of its archrival in the Clausura standings and seriously dented River's title hopes.
With five matches remaining, River has 22 points in seventh, with Boca on 21 in eighth. League leader Velez Sarsfield has 27 points.
Defeat also dumped River to fifth-bottom in the table of average points used to decide which clubs drop out of the top division.
The bottom two clubs are relegated automatically. The next two worst performers over a three-year period face playoffs with second division sides to determine their survival in the top flight.
River has not been relegated from the Primera Division since the average points system was first used in 1983.
David Ramirez twice gave Velez the lead with goals in the fourth and 29th minutes against Lanus, but Santiago Hoyos and Diego Valeri replied to leave the scores level at half time.
Silvio Romero got the winning goal in the 51st to move Lanus up to third position in the standings on 23 points.
Olimpo dropped to fourth after Facundo Parra and Patricio Rodriguez scored second-half goals in Independiente's 2-1 win.
Olimpo's Federico Dominguez was sent off in the 88th and a late goal from Julio Furch was scant consolation for the home side.
Godoy Cruz, which had a 2-0 home win against Quilmes on Friday, is second on 26 points.
Also Sunday, defending champion Estudiantes was held 2-2 by Tigre.
Gaston Fernandez opened the scoring with a penalty for Estudiantes in first-half stoppage time, before Ramiro Leone equalized in the 51st.
Enzo Perez gave Estudiantes the lead again on the hour mark, but Mariano Echeverria's 75th-minute strike ensured a share of the points for Tigre.
Both clubs remained mid-table with Tigre on 20 points and Estudiantes on 19.