Leaders Velez Sarsfield, fielding a number of reserves ahead of Thursday's second leg of their Libertadores Cup semi-final with Penarol, lost 2-1 at Tigre in the Argentine championship on Sunday.
Velez, 1-0 down to their Uruguayan opponents in South America's equivalent of the Champions League, fell behind to a first half penalty converted by Denis Stracqualursi, the division's top scorer with 10 goals in 15 matches.
Midfielder Ricky Alvarez equalised in the 62nd minute but Gaston Diaz restored Tigre's lead two minutes later to give second-placed Godoy Cruz, at home to Gimnasia-La Plata on Monday, the chance to catch Velez on 30 points at the top.
Velez have 30 points from 16 matches in the Clausura, second championship of the season, three points more than Godoy Cruz and four more than Lanus who are also in action on Monday at home to struggling Huracan.
River Plate were held 0-0 away to Olimpo in a match between two teams in the top eight places in the table but also fighting desperately to improve their three-season points average and avoid the relegation-promotion playoffs.
Boca Juniors, with a visit from former favourite Carlos Tevez at the Bombonera, beat Newell's Old Boys 1-0 with a 50th-minute strike by Martin Palermo, his fifth goal in six matches, making up for his first half penalty miss.
Tevez was given a rousing welcome from fans after clear-the-air talks on Saturday with Argentina coach Sergio Batista that may have reopened the Copa America door for the Manchester City striker.
Title holders Estudiantes lost 2-0 at home to Independiente to go 12 matches without a win in all competitions although they lead last weekend's match at Huracan 2-0 after it was abandoned due to crowd violence after half an hour. The remaining hour will be played behind closed doors on Thursday.
Estudiantes coach Eduardo Berizzo told reporters: "I might be on the bench on Thursday and I might not ... When one loses, the trainer is responsible."
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