Sunderland cannot afford to register anything less than victory at fourth bottom Norwich City next weekend after blowing the chance to close the gap on the Canaries.
Jamie Vardy's first goals in seven games were enough to secure a win for Leicester City and move them a step closer to a fairy-tale Premier League title.
But this was another of those familiar 'what might have been' games for Sunderland after substitute Jack Rodwell squandered a golden late chance to earn a share of the spoils and kept Sam Allardyce's men four points adrift of Norwich.
A point apiece would not have been an unjust outcome, with little between the teams in a game predictably full of endeavour and industry, due to their respective need for victory.
Leicester came out the traps instantly; hunting down in packs, winning the second balls and causing a few early scares, albeit the only effort on target was N'Golo Kante's routine half-volley from 25 yards.
However, skipper Wes Morgan should have made much more of a golden chance in the 15th minute after he was picked out by Christian Fuchs' left wing corner.
Morgan met the delivery with a free header at the near post, seven yards out, but it brushed off his temple and across the face of goal.
But Sunderland gradually rode the storm from Leicester's strong start, with Jan Kirchhoff and Yann M'Vila beginning to get a foothold in a breathless, physical encounter.
Opportunities remained few and far between for both sides - Younes Kaboul sending a header a foot over the bar from Fabio Borini's right wing cross.
Borini went closest to a first half breakthrough for either side in stoppage time when Jermain Defoe fed him the ball down the right-hand side of the area.
The Italian hit a low right-foot shot which deflected off Morgan, but was saved well by the legs of Kasper Schmeichel.
ithin six minutes of the restart, Borini went desperately close again when Defoe's shot deflected to him just inside the area, to the right of goal.
Borini had time to bring it under control and hit a right-foot shot just inches over the bar.
Sunderland then had an escape at the other end when Jamie Vardy lifted the ball over the top for Danny Drinkwater in the middle of the goal, but the midfielder's first touch was heavy and dribbled into Vito Mannone's arms.
Leicester were enjoying their best spell of the game, yet Sunderland had defended well until the 64th minute when the Black Cats were undone by a simple hoof over the top by Danny Drinkwater.
Vardy outpaced Kaboul as he raced onto it, before calmly placing the ball into the far corner from the left of goal.
Sunderland should have drawn level in the 82nd minute after a howler of a miss from Rodwell.
Patrick van Aanholt deflected a shot to the unmarked Rodwell, eight yards out, who just had to lift the ball beyond Schmeichel into the far corner, but he sent his shot a foot over the bar.
Mannone then prevented Leicester doubling their advantage at the other end when he saved at the feet of Leonardo Ulloa after the substitute was picked out on the counter-attack by N'Golo Kante.
The keeper then had to produce another excellent save in the last minute to keep out Vardy's low shot from Ulloa's pull-back.
Mannone was at it again in stoppage time to deny Daniel Amartey when he was clean through on goal.
But in the fifth minute of added-on time, Vardy did seal it - breaking beyond van Aanholt to leave himself with just Mannone to beat, before taking it around the keeper and tapping it into the empty net.
Sunderland: Mannone, Yedlin, Kaboul, Kone, van Aanholt, Kirchhoff, Cattermole, M'Vila (Rodwell 68), Khazri (N'Doye 68), Borini (Lens 76), Defoe. Subs not used: Pickford, Jones, O'Shea, Larsson. Booked: Borini (21)
Leicester: Schmeichel, Simpson, Morgan, Huth, Fuchs, Drinkwater, Kante, Mahrez (Gray 81), Albrighton (Amartey 84), Vardy, Okazaki (Ulloa 62). Subs not used: Schwarzer, King, Dyer, Chilwell. Booked: Fuchs (29)