Two goals from Wayne Rooney put Manchester United back on top of the Premier League for the first time in five months when they beat West Bromwich Albion 2-0 at Old Trafford on Sunday and Manchester City went down to a 1-0 defeat at Swansea City.
Rooney took his tally to nine goals from his last six matches and 26 for the season, opening the scoring with a close-range effort after 36 minutes, and he made it 2-0 when he converted a 71st-minute penalty after Keith Andrews hauled down Ashley Young.
The win lifted the champions on to 67 points from 28 games, one more than Manchester City who are second after leading the table since the middle of October.
Both teams were in action on Sunday after playing in the Europa League on Thursday when they lost the first legs of their round-of-16 matches.
United, surprisingly beaten 3-2 at home by Athletic Bilbao, bounced back immediately.
They already looked comfortable at 1-0 and the balance tipped their way even more when West Brom had Jonas Olsson sent off for a second bookable defence after tripping Javier Hernandez in the 66th minute.
Hernandez had gone close earlier for United, thumping a fine strike against the post.
City, who lost 1-0 in Lisbon to Sporting on Thursday, had a lucky escape at the Liberty Stadium when Scott Sinclair saw his seventh-minute penalty saved by Joe Hart.
City then had plenty of possession but could not find a goal.
With the game seemingly headed for a goalless draw that would have kept City top with a better goal difference than United's, the deadlock was broken seven minutes from time when Luke Moore, who had been on the field for four minutes after replacing Danny Graham, found himself unmarked in space and headed home Wayne Routledge's cross.
Micah Richards thought he had equalised for City in the dying minutes, but his effort was ruled offside.
Moore told Sky Sports News: "That was a massive win for us and our pressure paid off today. I don't score too many headers, but I'll take anything and it was a great cross from Wayne."