Afonso Alves sent an icy blast through Manchester United's hopes of wrapping up the title with an excellent double to earn Middlesbrough a share of the spoils in a 2-2 draw.
The result sees United's lead over Chelsea down to three points - and it still has to go to Stamford Bridge.
Gareth Southgate's side was well worth the point at a snow-swept Riverside, and could even have stolen a late winner.
United could point to several chances apart from Cristiano Ronaldo's 37th goal of the campaign and an equaliser from Wayne Rooney, but there was an unusual frailty in defence.
Perhaps the absence of Nemanja Vidic upset the balance - John O'Shea was never comfortable and Rio Ferdinand also limped off in the second half.
The biggest crowd of the season at the Riverside witnessed a hugely-entertaining encounter, Alves warming things up with a promising early run into the United penalty box.
The home fans were left cold however when Boro's marking completely went to pieces and United scored a 10th-minute opener.
First Michael Carrick found himself in the clear at the rear of the area to pick up possession from corner, and then Ronaldo drifted away from his marker to fire home his cross from inside the six-yard box.
Alves showed a couple of flashes, including a decent downward header, as Boro tried to reply then Ryan Giggs hit the side-netting after some stylish interplay between Ronaldo and Wayne Rooney.
Jeremie Aliadiere was then guilty of a horrendous miss after Middlesbrough broke at speed, the French striker pulling the ball wide from a one-on-one position.
Afterwards, United players complained bitterly that referee Mike Riley did not halt play to allow Patrice Evra to receive treatment - van der Sar even had to push Ronaldo away from the match official. Riley, though, was following the FA's rules to the letter and Evra was soon back on his feet anyway.
Ronaldo then showed human blood does flow through his veins after all. A thrilling run saw the Portuguese winger leave Gary O'Neil trailing in his wake but when he cut into the penalty box his finish for once lacked composure and was sliced wide.
Alves won a free-kick against Ferdinand 30 yards out and took it himself. It was a terrific effort two, curling and dipping wickedly but straight at van der Sar, who was up quickly again to claw away the follow-up header from Emmanuel Pogatetz.
But the Brazilian was not to be denied again and 10 minutes before the break he scored an excellent equaliser.
Gary O'Neil's long pass from the right caught United ball-watching, Aliadiere's flick-header took it into Alves' path and he kept his cool to lift the ball over the sliding van der Sar with his left foot.
Boro came agonisingly close to taking the lead when George Boateng found Stewart Downing on his own at the far-post but van der Sar was out like lightning to block the shot.
Ronaldo was then furious again when a powerful strike appeared to be blocked by Andrew Taylor's arm - Riley only awarded a corner.
Alves' equaliser had been the first goal conceded by United since February 23 so when he put Boro ahead in the 56th minute, the majority of the 33,952 crowd went mad with excitement.
Again Aliadiere played a part, the ball coming off his head into Alves' stride and the Brazilian