Chelsea have been a side with nothing to lose, while Manchester United's burden is defending a crown they have yet to win.
Chelsea and Man United in battle of great expectations
For years, the expectation has been that Chelsea's experience would prove their undoing. Instead, it has given them a stealth factor defending champions rarely enjoy.
Each pronouncement from Stamford Bridge was framed by years of knowledge-gathering. These were not wide-eyed innocents speaking as expectations were downgraded during their winter downturn.
"The gap is very difficult. We have to reach fourth place in the table," Carlo Ancelotti said in February.
A month later, and Didier Drogba's sights were set only marginally higher when the Ivorian stated: "We must now focus on third place."
And the title? "Technically, it is not over," Petr Cech said, while giving the impression that Chelsea's interest was only a technicality.
It might not even have been that. When Wayne Rooney put Manchester United ahead at Stamford Bridge on March 1, they possessed an 18-point lead in real terms. An hour later, goals from David Luiz and Frank Lampard had reduced United's advantage to 12 points.
Skip forward to today and it is three points. Should Chelsea win at Old Trafford, they will lead United on goal difference.
The great comeback is on.
This is a side with renewed belief and real momentum. The sight of John Terry celebrating wildly after Salomon Kalou scored the 89th-minute winner against Tottenham last week told a tale in itself: this was about more than a derby victory.
Meanwhile, even United may have been lulled into a false sense of security, taking comfort from every slip-up Arsenal endured. In December, Sir Alex Ferguson said five teams were in the title race. Two months later, he deemed it a two-horse race, with Arsenal's tendency to stumble over every hurdle easing United's passage to the title.
"Two or three weeks ago, they didn't think in this game they'd need to fight so much," Ancelotti said. "Mentally, for this reason, we can have an advantage."
But now United are on full alarm. A Champions League semi-final was demoted to second in the priority list this week when Ferguson opted to rest the majority of his preferred team against Schalke.
History presents a warning that Ferguson is heeding. Chelsea won at Stamford Bridge in the pivotal match of last season's title race four days after United faced German opposition (Bayern Munich) in the Champions League. The United manager has started to talk of the "emotional tiredness" such fixtures can produce. His hope is that rest lends a calm freshness.
If the trend in the Premier League is that United are being ruthlessly hunted down by Chelsea, these are two teams with different, but forward, momentum. Ancelotti's men have taken 25 points from nine league games, Ferguson's have reached a European final.
The quarter-final against Chelsea, Nani insisted, gives United the mental edge. "The results against them in the Champions League were very good," the winger said. "That gives us the psychological advantage."
Rio Ferdinand said: "Chelsea know we are well capable of beating them."
It is normally a given that United finish the season strongly, but strange shifts in impetus can harm as well as benefit. Last year's defeat to Chelsea, and the loss of the injured Wayne Rooney four days earlier, meant United were overhauled.
In 1998, as injuries became an increasing problem, a 12-point lead was surrendered as Arsenal became champions; two years earlier, however, United were inspired in pursuit, overcoming a 12-point deficit to overtake Newcastle United.
The situation is complicated by the respective expectations. Chelsea had none; they were a side playing with nothing to lose. United have been feted as champions elect and must react to a change in circumstances. Chelsea can pursue victory in the knowledge that nothing else is likely to be enough. United should find it hard to banish the thought that a draw would suffice.
For Chelsea, this week, like every other, is win or bust; for United it is the title-decider they may have thought they would not need. And whatever happens on the Old Trafford pitch, it is a game that could be won in the head.