Szczesny rushed out of his area to try and intercept but Niang got to the ball first, slipped it past him and scored into an empty net amid Polish disbelief.
Senegal are "not jumping up and down" after starting their World Cup campaign with a victory over Poland to revive memories of their run into the quarter-finals on their tournament debut in 2002.
Lewandowski has an enviable goalscoring record this season with 41 goals in 48 games in all competitions this season and will be looking to open his World Cup account against Senegal in Moscow.
Poland fans are crying foul after losing to Senegal on a sneaky goal by a player who seemed to come out of nowhere.
He sparked numerous Senegal attacks and capped off an impressive performance with a goal on the hour-mark.
Aliou Cisse, the Senegal coach's confidence was not misplaced.
The Senegal head coach explained that the match against Poland was hard as they were one of the best teams in Europe.
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Despite Krychowiak's late reply, Poland never really looked like snatching a point and must now pick themselves up for the match against Colombia on Sunday.
The youngest coach here at 42, Cisse said: "They had more of the ball but every time they made a mistake we hit them and forced them back". "People do believe in our team, and they are proud and we are also proud to represent Africa".
The clash between them will take place on June 24 and you can expect the stadium to be very tidy after.
The first shot on target produced the first goal as Senegal went ahead in the 37th minute.
Not only is Krychowiak built like a bulldozer, he moved like one too and Sadio Mane, Mbaye Niang and Idrissa Gueye enjoyed miles of space once they completed the simple task of slipping past the Poland man.
"We didn't get any good forward balls to do anything with, and instead concentrated on keeping the ball at the back and keeping possession".
Six minutes after the half-time, Poland was about to equalise the matter as a great free kick by Robert Lewandowski was saved by Senegal's keeper Khadim N'Diaye, who dived near the left post to send the ball for a corner.