* Boca four points behind San Lorenzo
* Second-placed Newell's Old Boys go sixth match without win
* Racing beat River 1-0 in derby between ailing big guns (Adds quotes, late result)
BUENOS AIRES, Nov 25 (Reuters) - Boca Juniors all but blew their chances of winning Argentina's league championship on Sunday when they were upset 2-0 by troubled All Boys at La Bombonera.
Boca's second consecutive defeat left them on 27 points, four behind leaders San Lorenzo with two matches to go in the Inicial championship.
A tight group of teams occupy the leading places with the Saints top on 31 points, Newell's Old Boys with 30, Lanus and Arsenal with 29 and Boca and Velez Sarsfield on 27.
Newell's went a sixth match without a victory, and have picked up a mere four points from the last 18 available, but are still very much in line to retain the title as their challengers have failed to capitalise.
Boca, who feted the world's top two tennis players Rafa Nadal and Novak Djokovic, in town for an exhibition match, on the pitch at halftime, fell behind to two goals early in the second half.
Midfielder Roberto Battion headed All Boys into the lead in the 47th minute and striker Mauro Matos converted a penalty in the 54th to take his tally as the championship's top scorer to nine.
All Boys went into the match under a new coach, Ricardo Rodriguez, who has extensive experience in the second tier Primera B Nacional, after former Julio Cesar Falcioni resigned midweek.
"This team needed a victory of this magnitude," Rodriguez told reporters.
The win helped keep All Boys one place above the bottom three in the relegation standings, which are decided on teams' average points over three seasons.
Boca goalkeeper Agustin Orion told Futbol Para Todos his team's chances were fading fast and said players must improve to justify their place at the club.
"Now we're a long way off," he said. "To stay here you have to win championships."
Asked if a spate of injuries was why Boca have struggled this season, Orion said: "That could be part of the reason but we need to improve, they were better than us today."
Argentina's Fernando Gago returned to the side after a leg muscle injury but midfielder partner Juan Roman Riquelme had to come off with a knock after 15 minutes.
San Lorenzo stayed top on Saturday when forward Ignacio Piatti scored twice in the final quarter of an hour to secure a 2-2 draw at Atletico Rafaela, who had led by two goals from the 50th minute.
Lanus, who had won their previous five league matches, were held 0-0 at home by Gimnasia.
Racing Club beat River Plate 1-0 in a derby between two of Argentina's so-called 'Big Five', both of whom are languishing in the lower reaches of the table.
Racing won at home for the first time in eight matches at the Cilindro through Bruno Zuculini's headed goal from a 15th minute corner. It was a Racing player's first goal at home this season. Their only previous marker at home came in a 1-1 draw with Lanus through an own goal.
River Plate, who began the championship with high hopes, went an eighth league match without a win.
(Reporting by Rex Gowar; Editing by Peter Rutherford)