FA Cup holders Leicester City suffered a shock fourth-round loss to east Midlands rivals Nottingham Forest in a pulsating game at the City Ground.
Forest, eighth in the Championship, took the lead with a well-worked goal that saw Keinan Davis head down Brennan Johnson's cross and Philip Zinckernagel poking the ball past Danny Ward.
Within 30 seconds, the hosts had doubled their advantage, Johnson finishing after a terrible backpass from James Justin. It became 3-0 when returning captain Joe Worrall powered in a header at the back post from James Garner's corner.
Premier League Leicester were handed a lifeline when home goalkeeper Brice Samba made an error and that enabled Kelechi Iheanacho to roll the ball into an unguarded net, but they were not able to fight back.
Forest restored their three-goal advantage when Djed Spence raced through and slid the ball past Ward to set up a fifth-round home tie against fellow Championship side Huddersfield Town.
Most of the match was played in front of a superb atmosphere at a packed City Ground, although there was an unsavoury incident after Forest's second goal. A fan ran on to the pitch and confronted a number of players before being tackled by security and led away.
Leicester's FA Cup defence falls apart
It was only in May when Youri Tielemans scored a stunning 25-yard goal as Leicester beat Chelsea 1-0 at Wembley as the Foxes won the FA Cup for the first time in their history.
But Brendan Rodgers' side tamely fell at this hurdle as Leicester followed Arsenal out of the competition after the Gunners had lost 1-0 at Forest in the third round.
However, in front of a crowd of 28,762, Forest were superb. Into the fifth round for only the second time in 17 years, they were not flattered by the winning margin against a side 10th in the top flight.
It could have been a different game had Iheanacho scored inside the opening five minutes, but instead fired wide. Forest, backed by vocal support from their fans, then took control inside a crazy five-minute spell midway through the half.
This was the first meeting between the two sides in almost eight years, but it was hard to tell which one played in the higher division.
Davis showed superb technique to volley against the Leicester bar before Zinckernagel's opener, with the visitors gifting Forest a second goal only moments later.
Worrall, back after a three-game absence with broken ribs last month, scored Forest's third in the 32nd minute - capping a stunning nine-minute spell that took the game beyond Leicester.
Samba's error, when he charged out of his box to challenge Iheanacho, gifted Leicester a 40th-minute goal and the Foxes started the second half brightly with James Maddison and substitute Patson Daka missing chances.
But the excellent Spence killed the game off in the 61st minute, sprinting through and firing home Forest's fourth.
It followed more abject defending that Rodgers will be unhappy about as his side's cup defence ended in disappointment.
Line-ups
Nottm Forest
Formation 3-4-1-2
30Samba
4Worrall27S Cook26McKenna
2Spence22Yates37Garner15Lowe
11Zinckernagel
20Johnson9Davis
30Samba4Worrall27S Cook26McKenna2Spence22Yates37GarnerBooked at 65minsSubstituted forRibeiro Diasat 88'minutes15Lowe11ZinckernagelSubstituted forColbackat 63'minutes20JohnsonSubstituted forCosta Silvaat 88'minutes9DavisSubstituted forSurridgeat 71'minutesSubstitutes
1Horvath3Figueiredo8Colback14Laryea16Surridge18Ribeiro Dias19Costa Silva21Ojeda23LolleyLeicester
Formation 4-2-3-1
12Ward
2Justin18Amartey4Söyüncü33Thomas
8Tielemans25Ndidi
37Lookman10Maddison7Barnes
14Iheanacho
12Ward2Justin18Amartey4Söyüncü33Thomas8TielemansSubstituted forDewsbury-Hallat 65'minutes25NdidiBooked at 59mins37LookmanSubstituted forRicardo Pereiraat 65'minutes10MaddisonBooked at 74mins7BarnesSubstituted forDakaat 45'minutes14IheanachoSubstitutes
1Schmeichel11Albrighton17Pérez20Choudhury21Ricardo Pereira22Dewsbury-Hall23Vestergaard29Daka42SoumaréReferee:Paul TierneyAttendance:28,762Live Text
Match ends, Nottingham Forest 4, Leicester City 1.
Second Half ends, Nottingham Forest 4, Leicester City 1.
James Maddison (Leicester City) wins a free kick on the right wing.
Foul by Max Lowe (Nottingham Forest).
Attempt missed. Ricardo Pereira (Leicester City) right footed shot from the right side of the box misses to the left. Assisted by Patson Daka.
Foul by James Maddison (Leicester City).
Djed Spence (Nottingham Forest) wins a free kick in the defensive half.
Substitution, Nottingham Forest. Xande Silva replaces Brennan Johnson.
Substitution, Nottingham Forest. Cafú replaces James Garner.
Attempt missed. James Garner (Nottingham Forest) right footed shot from outside the box misses to the left.
Foul by Daniel Amartey (Leicester City).
Sam Surridge (Nottingham Forest) wins a free kick in the defensive half.
Foul by Çaglar Söyüncü (Leicester City).
Sam Surridge (Nottingham Forest) wins a free kick in the attacking half.
Attempt saved. Daniel Amartey (Leicester City) left footed shot from outside the box is saved in the bottom left corner.
Corner, Leicester City. Conceded by Jack Colback.
James Maddison (Leicester City) wins a free kick on the left wing.
Foul by Djed Spence (Nottingham Forest).
Hand ball by James Maddison (Leicester City).
Foul by James Maddison (Leicester City).