Manchester United's sizzling start to the Premier League season faces its sternest test Sunday against Chelsea.

The defending champions, who are chasing their 20th English title, have opened the season by scoring 18 goals in its four victories.

Last year, Chelsea opened its title defense with five wins before a slump saw the London club surrender the trophy to United and cost manager Carlo Ancelotti his job.

''This is the first big, big test of our team and how we've improved, and we're going there trying to show our best football and to secure a good result,'' Chelsea defender Branislav Ivanovic said. ''United have started very well this season and have improved as a group, but, if you remember, we started last season the same way. It will be the hardest game for us until now.''

United is above neighbor Manchester City on goal difference, while Chelsea - now managed by Andre Villas-Boas - is two points behind in third.

While Chelsea striker Fernando Torres struggles for goals - just one since signing in January - United's Wayne Rooney has scored eight this season alone, including hat tricks in the last two league matches.

''We face United perhaps in their most tremendous moment of motivation, flair and style,'' Villas-Boas said.

But while Chelsea opened its Champions League campaign with a 2-0 victory over Bayer Leverkusen on Tuesday, United could only draw 1-1 at Benfica on Wednesday.

Man City was also given a reality check in Europe, drawing 1-1 at home with Napoli on Wednesday in its Champions League debut. But Roberto Mancini's big-spending team has been almost as rampant as United on the domestic front, scoring 15 goals in its four victories.

Still, Mancini expects to see an improvement at Fulham on Sunday.

''It is important for us to play simple football,'' Mancini said. ''If we play simple I think we can play better, score goals and not concede any chances.''

While it is to surprise to see United, City and Chelsea occupying the top three places, Newcastle and Stoke are next after bright starts.

Newcastle is at Aston Villa on Sunday and Stoke will be looking to extend its unbeaten start at Sunderland the following day. Also Sunday, sixth-place Liverpool is at Tottenham.

Liverpool defender Daniel Agger has described the belief within the squad as being ''miles apart'' from how it felt at this time last season, when the club was near the bottom of the standings.

''If you look at the squad now, it looks really, really strong with so many good players so I think it is good for us to have the belief to go forward and win games,'' Agger said. ''We definitely have that now.''

Arsenal is still trying to rediscover that belief after collecting just four points in four matches this season and drawing 1-1 at Borussia Dortmund in the Champions League on Tuesday.

The Gunners head to Blackburn on Saturday with their new players still adjusting.

''With a club like Arsenal, you are going to be under pressure and you need to handle it and try your best,'' Arsenal midfielder Mikel Arteta said. ''Most of the new players came in at the last minute so its not going to happen in the first minute but when you have got the quality, it makes things easier.''

Blackburn is bottom of the standings with just one point from four matches.

Also Saturday, Swansea hosts West Bromwich Albion, Bolton takes on Norwich and Wolverhampton Wanderers faces Queens Park Rangers.