Fulham won a seventh home league game in a row for the first time since 2000 to reinforce their place in the top six as Villa dropped out of the automatic promotion places.
Ryan Sessegnon and Floyd Ayite were on target as the Cottagers became the first team to inflict a league defeat on Villa in 2018.
Victory was effectively sealed on 71 minutes for the home side, although it came from an very bad error by Villa keeper Johnstone.
Adomah has weighed in with 14 goals in all competition for the club this season while teammate Grealish has been in excellent form for Villa since his return to the side following injury late a year ago.
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Sean Morrison headed the victor after 34 minutes to seal all three points at the Cardiff City Stadium.
Fulham pulled themselves within four points of Villa with a deserved victory.
Meanwhile Fulham are now 10 games unbeaten and up into fifth.
John Welsh was dismissed for a second yellow card and that opened the door for the visitors to equalise nearly immediately through Helder Costa.
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Chris Coleman's men sit 23rd in the table going into Saturday's home game against Brentford and it's nearly at the stage where they can ill-afford to keep losing at the Stadium Of Light, especially given their poor away form that has seen them concede 10 goals in their last three matches.
Both players lasted the majority of Villa's last match against bitter rivals Birmingham City at Villa Park last weekend before being substituted with just minutes remaining. We had one or two opportunities in the first-half where if we had been brighter.
Forest had to play for more than an hour a man down after Eric Lichaj was given a straight red card for a poor challenge on Albion attacker Martin Samuelsen on 28 minutes.
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