A blunder by goalkeeper Oscar Ustari paved the way for Boca Juniors' 2-0 defeat at Lanus that allowed Newell's Old Boys to overtake them as Argentine championship leaders on Sunday.
It was a fine result for Lanus coach Guillermo Barros Schelotto, a Boca idol in his days as a winger who masterminded the victory against his old club.
"I didn't enjoy it fully like I would beating another team," the former Columbus Crew forward told reporters. "We were always in control, we deserved to win."
Ustari spilt a high ball into his penalty area, defender Rolando Schiavi's attempted clearance hit the bar and striker Mario Regueiro put Lanus ahead after half an hour.
Despite Uruguayan Regueiro's red card in the 81st minute for a second booking, Lanus increased their lead in the 86th through Diego Valeri in a counter-attack.
Former Paraguay World Cup coach Gerardo Martino's unbeaten Newell's team, who won 2-1 at Colon, lead the Inicial championship standings with 18 points from eight matches.
Velez Sarsfield, who won 2-1 at San Lorenzo on Saturday, have 17 points and Boca 16.
Big guns River Plate, Independiente and San Lorenzo are struggling in the lowers reaches of the standings, desperate for points early in the season to stave off the threat of relegation.
River lost 1-0 at home to Racing Club and are one from bottom of the relegation points averages with Independiente, who were held 1-1 at home by All Boys on Saturday, below them.
Fans are demanding a change of coach at River where they see the inexperienced Matias Almeyda struggling to build a solid team and chanted at the end of Sunday's defeat for the return of the highly successful Ramon Diaz.
Independiente, having handed the reins to Americo Gallego at the end of last month, have gone 15 matches without a league win.
San Lorenzo's coach Ricardo Caruso Lombardi, who normally finds something to complain to the referee about, tried to put a lid on his frustrations after a second successive defeat, saying: "I look like Snow White I'm so calm, but inside I'm a volcano.
"At big clubs, time is shorter [but] while I feel good with the players, I'll carry on. They support me."
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