Kemar Roofe scored twice as fourth-tier Oxford United pulled off the first major shock in this season's FA Cup by stunning Premier League stragglers Swansea City 3-2 on Sunday.
Chelsea avoided the same fate, beating third-tier Scunthorpe United 2-0, while Tottenham Hotspur needed an 89th-minute penalty from Harry Kane to salvage a 2-2 draw at home to Leicester City.
Swansea went ahead at Oxford through Jefferson Montero's clever back-heel, but Liam Sercombe equalised and Roofe's classy 10-minute brace took the League Two side into round four despite Bafetimbi Gomis replying in the 66th minute.
"It's going crazy out there. Everyone is bouncing. The atmosphere was brilliant and it's the same in the dressing room now," Roofe, who came through the youth ranks at West Bromwich Albion, told BBC Sport.
"We're probably used to more possession in the league and obviously we came up against a well-known passing team, but we matched them and got in their faces.
"We'll enjoy the moment tonight, maybe have the day off tomorrow, and get back on it the day after."
Swansea manager Alan Curtis, who was this week placed in charge until the end of the season following the dismissal of Garry Monk, gambled by making 10 changes to his starting XI at the Kassam Stadium.
He saw his team go ahead with a beautifully constructed goal, Montero darting in from the left, exchanging passes with Marvin Emnes and beating goalkeeper Sam Slocombe with a deft, improvised finish.
But Oxford equalised in the 45th minute when Sercombe drilled home from the spot after Kyle Bartley was penalised for diving in on Alex MacDonald.
Roofe, who turned 23 on Wednesday, put Oxford ahead four minutes after the break, curling a shot past Kristoffer Nordfeldt from outside the box, and 10 minutes later he claimed his second goal after a swift Oxford break.
Gomis reduced the arrears with a cool finish from Jack Cork's disguised pass, but Oxford, League Cup winners in 1986, withstood late pressure to claim one of the most famous results in their history.
- Loftus-Cheek opens account -
"We made changes and there was a certain amount of rustiness, but I am disappointed," said Curtis, whose side are currently 17th in the Premier League table.
"The league was always going to be the priority, but we wanted a cup run and to get some momentum going. It's a huge setback for the club."
Tottenham also looked to be heading out of the tournament after Marcin Wasilewski and substitute Shinji Okazaki struck to put Leicester 2-1 up at White Hart Lane, Christian Eriksen having earlier opened the scoring.
But with two minutes remaining, Leicester winger Nathan Dyer was penalised for handball and Kane, who had come on in the 68th minute, slammed home the ensuing penalty to keep Spurs in the cup.
The teams resume hostilities in the Premier League on Wednesday, when fourth-place Spurs host second-place Leicester.
"Today is a good example of not giving up and fighting," said Spurs manager Mauricio Pochettino.
"We feel disappointed, but that is football and we have the opportunity to go to Leicester and still be in the next round."
Chelsea interim manager Guus Hiddink's quest to repeat his feat of 2009 by leading the club to FA Cup glory got off to a positive start with a routine home win over Scunthorpe.
Diego Costa steered in Branislav Ivanovic's cross to give Chelsea a 13th-minute lead and half-time substitute Ruben Loftus-Cheek, 19, clipped home his first goal for the club from Cesar Azpilicueta's pass in the 68th minute.
There were no major upsets in Saturday's third-round action, but Manchester United needed a stoppage-time penalty from Wayne Rooney to earn a 1-0 win over third-tier Sheffield United.
Holders Arsenal, bidding for a third consecutive success in the competition, and Manchester City went through, but Liverpool were taken to a replay by fourth-tier Exeter City after a 2-2 draw on Friday.