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United trail leaders Manchester City, who host struggling Bolton on Saturday, by two points going in to the weekend.
Spurs' own push for the top has come unstuck in recent weeks but they still represent a major test to United.
Ferguson believes that a win at White Hart Lane and keeping up with City at this stage could prove decisive, with United facing a game at City in the third last fixture of the season.
He said: "It's a very, very important game for us. If you look at the rest of the season you would certainly say that building up to a game that could turn out to be the decider against Manchester City later in the season, Tottenham is our hardest away game but we'll be looking to navigate it.
"I'm looking at Manchester United. We realise how important it is on Sunday to maintain our challenge.
"We're not interested in anyone else's challenge but to maintain our challenge we need to win on Sunday."
Ferguson feels that Tottenham managing to fight off Chelsea's interest in Croatian midfielder Luka Modric last summer has been key to their successful campaign.
The playmaker made it clear that he wanted to leave but Spurs rejected a string of offers from Chelsea.
Ferguson said: "I think it swung the players around too that they decided to hold on to Modric against what we thought was a big Chelsea bid and it's an indication that they mean business.
"It's worked because they've had a great season and up until about three weeks ago they were playing the best football in the league.
"They've hit a little bit of a plateau at the moment but we know that going down there on Sunday is a big game for us and it's the most important game for us so far."
Ferguson has a doubt over defender Chris Smalling, who sustained a head injury in England's 3-2 friendly defeat to Holland in midweek.
Wayne Rooney has trained all week after missing international duty and United's previous two matches due to a throat infection.
Antonio Valencia and Tom Cleverley remain on the sidelines but Michael Owen has returned to training after being out since November with a thigh injury.
Ferguson added: "Michael Owen has started training which is good news. Hopefully the lad has a bit of luck because he's not had much at all.
"When he gets an injury it's always a long-term one but hopefully he's back for the rest of the season which will help us."
Ferguson believes that the contribution of Michael Carrick has once again been vital to United's challenge for the title.
Carrick, 30, joined from Tottenham in 2006 and has won four league titles in his five seasons at Old Trafford as well as helping United to three Champions League finals in four years.
Ferguson has always claimed that Carrick is a slow starter but feels that the player's partnership with Paul Scholes has been key to United keeping pace with City.
