RIO DE JANEIRO, April 8 (Reuters) - Titleholders Boca Juniors slumped to a 3-1 defeat at Atlas in the Libertadores Cup on Tuesday after an extraordinary miss by striker Martin Palermo when the scores were still level.
The defeat in Guadalajara, Mexico, left Boca, Argentina's most popular club, in danger of failing to reach the last 16.
Mexico's Atlas went ahead when Gerardo Flores pounced on slack defending to score from close range in the 21st minute.
Sebastian Battaglia equalised nine minutes later and Leandro Gracian squandered an excellent chance to put Boca in front before Palermo, the club's all-time leading scorer, produced a candidate for the miss of the season.
Unmarked at the far post and needing to tap the ball into an empty net from six metres, Palermo, who famously missed three penalties for Argentina against Colombia at the 1999 Copa America, side-footed wide.
Atlas, backed by 60,000 crowd including a fanatical hardcore mass of jumping, shirtless fans behind the goal, went ahead following another Boca mishap eight minutes after the restart.
Defender Jonatan Maidana attempted to cut out a cross but teed the ball up for Bruno Marioni, who rifled it into the roof of the net.
Marioni, a fringe player for Boca last year, added the third on the break in the 75th minute.
Atlas went top of Group Three with 10 points from five games, followed by Boca on seven. Both teams have one game to play.
Chilean champions Colo Colo have six points but a game in hand. The top two qualify.
Colombian provincial side Cucuta went through to the next round with a 4-2 win at Bolivian champions San Jose in Group Six.
Striker Matias Urbano scored a hat-trick for the visitors, who made light of the difficulties of playing at 3,700 metres above sea level in the mining town of Oruro.
Urbano scored in the 35th and 38th minutes and made the points safe in the 78th.
Luis Cerutti pulled one back for San Jose in first-half stoppage-time, midfielder MacNelly Torres restored Cucuta's two goal lead in the 53rd minute and Enrique Parada pulled another back in the 77th.
Cucuta have 11 points from five games, four ahead of Brazil's Santos who have a game in hand. San Jose have four points from five games and Mexico's Guadalajara have three from four.