Chelsea staged a dramatic second-half fightback to rescue a 2-2 draw at Manchester United.
Rafael Benitez's Cup holders were set to surrender the trophy after falling two goals behind within the first 11 minutes.
Javier Hernandez got the first for United with a looping header and Wayne Rooney, back in the starting line-up after being controversially dropped in the midweek Champions League defeat against Real Madrid, netted the second with a curling free-kick.
However, Eden Hazard came off the bench to give Chelsea a lifeline with a fine finish in the 59th minute and Brazil midfielder Ramires slotted home the equaliser nine minutes later to set up a replay at Stamford Bridge.
In Saturday's quarter-finals, Manchester City had routed Championship side Barnsley 5-0 at Eastlands, while Wigan enjoyed a surprise 3-0 win at Everton.
Earlier, Blackburn earned a replay also against Millwall after riding their luck in a 0-0 draw at the Den on Sunday.
Michael Appleton's team were second best for long periods of the last eight clash between two out of form Championship clubs, but they enjoyed their share of good fortune to set up a replay at Ewood Park on Wednesday.
Andy Keogh's first half header struck a post before Lions striker Rob Hulse missed an absolute sitter from close-range in the second half.
Rovers, owned by Indian poultry firm Venky's, have endured a troubled season in the second-tier after last year's relegation from the Premier League and Appleton is Blackburn's third manager of the campaign.
They didn't have a shot on target against Millwall, yet are still in with a chance of a Wembley semi-final appearance thanks to their escape in south London.
"It's always going to be a battle when you come to the Den with the Millwall fans behind them," Appleton said.
"I was delighted with the way the lads went about it. I had three or four players who were only 60-70 per cent fit and we've got another chance to get through.
"The game on Wednesday is going to be a completely different game."
Millwall manager Kenny Jackett added: "If you don't score you're not going to win games. We did a lot of things right, put a lot of pressure on them and had clear-cut chances without scoring.
"That's been us of late. We need to find the quality to put the ball in the net. That's the thing that is lacking at the moment."