Sir Alex Ferguson says Manchester United are lucky to still be in the FA Cup after a resurgent Chelsea nearly snatched victory at Old Trafford.
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If United beat Chelsea in the replay, they will face Manchester City in the semi-finals in April.
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But Ferguson said his team paid for their efforts against Real Madrid.
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"We're lucky to be in the cup. We just ran out of legs and Chelsea were far the better team in the second half," Ferguson said after the 2-2 draw.
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"But I can't be critical of my side because the running they had to do on Tuesday night, against a really top side in Real Madrid, had its bearing on today's result, no doubt about that.
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"The two full-backs, Rafael and Patrice [Evra], are knackered and the two central midfielders tired badly. We kept giving the ball away and it made it a long day for us, going from one penalty box and back to our own."
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Chelsea recovered from a poor first half as Ramires and Eden Hazard levelled the game after the break following early goals from Javier Hernandez and Wayne Rooney.
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And, with barely a minute remaining, United had goalkeeper David De Gea to thank after he denied Spanish compatriot Juan Mata with a superb save.
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That stop prevented Ferguson's side going out of two competitions in a week, following their controversial Champions League defeat by Real Madrid.
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Rooney was dropped from that game, but returned to the starting line-up against Chelsea - though he was one of several players to fade in the second half.
Fergie is absolutely right. Luck earned us the replay we got. Chelsea could have finished us