Alex Ferguson predicts a long, hard Premier League title race with champions Manchester City after his Manchester United team opened up a seven-point gap over rivals Chelsea.

Table-toppers United ground out a 1-0 victory over West Ham at Old Trafford thanks to Robin van Persie scoring the fastest goal of the Premier League season to date, after only 33 seconds.

It was only the third clean sheet United have kept in domestic football this season and ended a worrying trend of the team having conceded first in nine of their opening 13 league games.

And while it was a far from vintage performance, it was a vital one given that City's victory at Wigan kept the defending champions just one point behind.

"The three points means we're still ahead in the league and, with Chelsea drawing, it draws us away from them a little bit further," said veteran United manager Ferguson. "But City are on our tails. It's going to be a long season.

"The importance of winning 1-0 in certain games is your opponents know the resilience is there.

"If you look at some of the really important one-nils we've had, go back to beating Newcastle when they were nine points clear of us (in 1996).

"Our 1-0 that night won us the league. There are important 1-0 victories. We've had many of them."

Given the scarcity of clean sheets for United this term, the fact his side were able to keep West Ham at bay for such a long period gave Ferguson as much pleasure as the large points cushion he enjoys over European champions Chelsea and their new manager Rafael Benitez.

"It is particularly hard, when you take the lead after one minute, to keep a clean sheet over the remaining 90-plus minutes, with time added on," said Ferguson. "But in the main I thought we defended very well."

Ferguson paid particular tribute to young Brazilian full-back Rafael and fit-again central defender Chris Smalling, who was starting a game for just the third time this season.

"Chris came into the team and did very well after a few weeks out and Jonny Evans was magnificent," he said. "There were a lot of plus points tonight, particularly with the defending because you have to against West Ham because they are such a handful. We had to be at our best for the whole 90 minutes.

"Rafael is maturing, he's 22 years of age now. He came here as a young boy, he was in the first team at 17-18, and made a lot of rash mistakes because he was such an enthusiastic boy. He is maturing and learning now."

Van Persie now has nine goals in just 12 league starts this term and Alan Shearer and Cristiano Ronaldo's joint record for a 38-match Premier League season -- 31 goals -- will be under threat if he maintains this form.

West Ham manager Sam Allardyce certainly believes that, whether van Persie sets records or not, his goals could see United to the title.

"He's going to score 20-odd, maybe 30 goals," he said. "In the end, that's normally the difference, if you sort your defensive side out.

"It's not so long ago, a couple of seasons, that Man United went 14 games without conceding a goal and that won them the league.

"Van Persie is a quality player, no doubt about that, though we kept him quiet tonight.

"United have got lots of players who will have good spells, lot of players who will come up trumps, and that's what makes them so good."