OXFORD - 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 against third-tier Scunthorpe United, with goals from Diego Costa and Ruben Loftus-Cheek earning them a straightforward 2-0 win.

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 through 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.

"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." 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 League One Scunthorpe. Costa steered in Branislav Ivanovic's cross to give Chelsea a 13th-minute lead and half-time substitute Loftus-Cheek, 19, clipped home his first goal for the club from Cesar Azpilicueta's pass in the 68th minute. Hiddink praised Loftus-Cheek for "a lovely, clean finish". "He can be of big value now," added the Dutchman. "He's 19. I thought he was 22, 23 when I came in. He's a proper grown player already." 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. Tottenham Hotspur host Leicester City later on Sunday.