Wayne Rooney scored his first goal from open play since March to help United to a 2-1 win at West Bromwich Albion, and Adam Johnson gave City a 1-0 win over Blackpool.
The Manchester teams each have 41 points, although United has played two games fewer than its rival's 21. Arsenal can stay just two points back if it beats Birmingham in the day's late match.
Tottenham moved back into fourth place after Gareth Bale's 11th goal of the season earned a 1-0 win over Fulham, but Chelsea is likely to move back into the final Champions League spot when it hosts ailing Aston Villa on Sunday.
Joe Cole scored his first Premier League goal for Liverpool in the last minute for a 2-1 win over Bolton that should lift some of the pressure on manager Roy Hodgson, while West Ham finally made it out of the relegation zone with a 2-0 win over Wolverhampton Wanderers.
Rooney opened the scoring for United in the third minute when he headed in Patrice Evra's cross, but James Morrison equalized 11 minutes later with a long-range shot.
West Brom then thought it should have had a first-half penalty when referee Chris Foy turned down appeals for a foul by Gary Neville on Graham Dorrans. But striker Peter Odemwingie wasted the penalty Foy did award, putting a weak kick wide in the 63rd after United defender Rio Ferdinand had fouled Jerome Thomas.
Hernandez came on as a 61st-minute substitute for leading scorer Dimitar Berbatov and headed in unmarked from a corner to clinch an 11th win from 19 games.
“You've got to win games. I don't care how we win. The three points is all that counts,” Ferdinand said. “You've got to dig deep in these games and these are the type of games you can make up a lot of ground in the league.” The only unbeaten side in the 20-team league, United is chasing a 19th English title to beat the record it shares with Liverpool.
Liverpool is back in ninth place but things could have been much worse for the five-time European champions, who lost 1-0 to Wolves on Tuesday.
Kevin Davies put Bolton ahead two minutes before halftime before Fernando Torres, who has been far below the form that brought him 72 goals in the past three seasons, equalized with just his sixth goal of the campaign in the 49th.
Former England midfielder Cole, who joined after he was released by Chelsea at the end of last season, then tapped in from close range with seconds left.
City could have further improved its goal difference as it tries to keep pace with United, but Carlos Tevez hit a 37th-minute penalty wide. Still, Johnson had scored just three minutes earlier with a deflected shot from the edge of the area.
Tottenham owed its third straight win to Bale's quick reflexes.
The winger headed Rafael van der Vaart's 42nd-minute free kick, which the Netherlands international had apparently intended as a shot, from a crowd of players past stranded goalkeeper Mark Schwarzer.
Spurs, who finished in the top four last season for the first time in 20 years, have 36 points. Chelsea can move up to 37 if it wins Sunday, when Wigan plays Newcastle.
Elsewhere Saturday, Darren Bent's eighth Premier League goal of the season helped Sunderland to a 3-0 win over Blackburn and Stoke beat Everton 2-0.
West Ham's 2-0 win over Wolves pushed its opponents into last place. Wolves defender Ronald Zubar scored a 51st-minute own goal and Freddie Sears completed the scoring in the 79th to take the Hammers from 20th to 15th place.
It's the first time West Ham manager Avram Grant has been out of the relegation zone since November 2009. Grant spent much of last season in the bottom three with Portsmouth, which reached the FA Cup final but was relegated.
