Luciano Becchio's well-worked goal had taken Leeds into the break 1-0 ahead, but Juan Mata equalised within two minutes of the restart.
Two goals in three minutes from Branislav Ivanovic and Victor Moses then effectively sealed Chelsea's passage into the last four, although the final scoreline was harsh on Warnock's team with Eden Hazard and Fernando Torres on target late on.
"To concede as early as that in the second half against world-class players you're going to be in trouble," Warnock said.
"It was such a relief from them to get back into the game. Then they took over and were too good for us."
Warnock admitted that goalkeeper Jamie Ashdown, who made several important stops in the first half, was at fault for Mata's goal, letting his shot slip through his grasp and into the corner of the net.
"When you're a goalie you make a mistake and it counts," he conceded. "When you're anywhere else on the pitch you get away with it.
"I don't think the scoreline reflects the game but I've got no complaints. I thought it was a great cup tie for an hour."
Much of the pre-match build-up involved specualtion about whether Warnock would snub opposite number Rafael Benitez, in a row going back to the days when the pair were managers of Sheffield United and Liverpool, respectively.
But the pair shook hands before and after the game and the Yorkshireman joked: "If it wasn't for Rafa I wouldn't have had a great time at Crystal Palace, at QPR and now at Leeds, so I owe him one really, don't I?"
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