Carlos Tevez's late equaliser kept Manchester United on course for the Premier League title as they earned a crucial point at Blackburn.
Roque Santa Cruz threatened to blow the race for the championship wide open with a calm finish after 21 minutes.
And Blackburn keeper Brad Friedel kept United at bay almost single-handedly with a string of stunning saves.
Tevez's 88th minute header gives United a three-point lead over Chelsea before their visit to Stamford Bridge.
United boss Sir Alex Ferguson made four changes ahead of the Champions League semi-final against Barcelona - with Nemanja Vidic returning after a three-game absence because of a knee injury and Tomasz Kuszczak stepping in for Edwin van der Sar in goal.
It contributed to moments of uncharacteristic uncertainty at the back, with Kuszczak having to rush out to block Jason Roberts from point-blank range after six minutes.
Tevez wasted a glorious chance to give United the lead three minutes later when he failed to apply the finishing touch to an inviting cross from Wayne Rooney.
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And Blackburn made them pay after 21 minutes when Morten Gamst Pedersen's throw led to a mix-up between Vidic and Rio Ferdinand, leaving Santa Cruz to slide in his 20th goal of the season from 12 yards.
David Bentley then forced Kuszczak to turn over his 25-yard shot as Blackburn grew in confidence after taking the lead.
Rooney demonstrated the lack of discipline that still dogs his game when he picked up a deserved yellow card six minutes before the interval.
He was still seething after being refused a penalty by referee Rob Styles when he tumbled under a challenge from Steven Reid.
And it was all too inevitable that Rooney would pursue retribution almost instantly, launching a dangerous two-footed tackle on Christopher Samba that was lucky to be only punished with a booking.
There was more frustration after 42 minutes when Ronaldo rose to meet Ryan Giggs' corner, only to see Friedel produce a fine diving save.
It was Giggs' final contribution as he was replaced by fit-again Nani during the interval.
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United, predictably, upped the tempo after the break and they were desperately unlucky not to draw level after 63 minutes when Ronaldo sent an angled long-range drive against an upright.
But they survived a scare 10 minutes later when Pedersen drove into the United area and flashed the ball across the face of goal with no-one able to make contact.
Rooney appeared to be struggling with a hip injury, but he was still a threat and tested Friedel with a powerful low shot.
Blackburn somehow survived again with nine minutes left when Rooney set up Ronaldo, who was denied by Johann Vogel's tackle. Tevez turned the loose ball towards goal, but Friedel got a hand to his effort and then snatched the ball off the goal-line.
And United looked to be denied a penalty when Michael Carrick's goal-bound header clearly struck Brett Emerton on the arm, but once again their claims fell on deaf ears.
Blackburn were under fierce pressure, and once more Friedel was the hero with six minutes left, saving brilliantly from Rooney when he was clean through.
Friedel was turning in an inspirational performance, and he somehow denied substitute John O'Shea from almost on the line as United laid siege to Blackburn's goal.
But he could do nothing as United finally got the equaliser they deserved with two minutes left.
Nani's corner was flicked on by Paul Scholes and Tevez headed in from six yards to send the United's hordes wild in celebration.
Blackburn: Friedel, Emerton, Samba, Nelsen, Warnock, Bentley, Reid, Vogel (Mokoena 85), Pedersen, Santa Cruz, Roberts.
Subs Not Used: Brown, Ooijer, McCarthy, Dunn.
Booked: Reid.
Goals: Santa Cruz 21.
Man Utd: Kuszczak, Brown (O'Shea 81), Ferdinand, Vidic, Evra, Ronaldo, Carrick, Scholes, Giggs (Nani 46), Tevez, Rooney (Park 90).
Subs Not Used: Foster, Pique.
Booked: Rooney.
Goals: Tevez 88.
Att: 30,316
Ref: Rob Styles (Hampshire).
BBC Sport Player Rater man of the match: Blackburn's Roque Santa Cruz 7.77 (on 90 minutes).
• Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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