Demba Ba's spectacular 49th-minute volley and a stunning Petr Cech save earned Chelsea the win at Stamford Bridge to set up a Wembley semi-final with Manchester City on April 14.
Benitez said: "The race for the top four will be until the end of the season. We're in the top four with one game in hand so we're in a very good position. We're in the semi-finals of the FA Cup, quarter-finals of the Europa League. It's a great season, at the moment. It could be even better."
In a match of few chances, Chelsea took the lead when Ba escaped the attentions of Rio Ferdinand to meet Juan Mata's lofted pass and find the top corner. United had plenty of time to find an equaliser. Javier Hernandez had an opportunity to level when he met a Danny Welbeck cross, but Cech was somehow able to divert the ball over.
Benitez added: "It was a great save, with a great goal."
Substitute Robin van Persie spurned two late chances as Chelsea ended United's hopes of a first FA Cup trophy since 2004 and a first double since the 1999 treble-winning season.
Sir Alex Ferguson felt Cech's save was the key moment. The United boss said: "Cech's save from Chicharito was unbelievable. He made a similar save in the first (game) from one of his own defenders.
"I think if we had got that goal then we would have been okay. It may have gone to extra time, of course, but it would have brought us back into the game and we didn't need to go chasing it."
Ferguson, who was without Wayne Rooney due to a groin injury sustained on England duty, was unimpressed with the performance of his side.
"Over the 90 minutes only Antonio Valencia reached the standard we expect of them," Ferguson said. "Phil Jones (did) for a good part of the game, of course, and David de Gea did well, but in terms of the 90-minute performance Antonio Valencia was the only one."
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