An attendance of 32,277 saw Boro cruise to victory against 10-man Nottingham Forest, who have now gone five games without a win.
Goals from George Friend, Jelle Vossen and Grant Leadbitter ensured the Teesside club bounced back from their defeat to Ipswich last weekend to remain fourth in the Championship.
“It’s my second Boxing Day here and both of them have been brilliant for me,” said Karanka.
“It’s good for us because it was a difficult game. My players are used to playing in front of 12,000 to 16,000 people.
“I don’t think many of them are used to playing in front of 32,000 people and in that kind of atmosphere.
“So they did very well. We finished the first half much better than we started. I’m very pleased because I wanted a good reaction after the Ipswich game.”
The opening goal came in the 54th minute when Friend leaped high to head Leadbitter’s corner beyond Forest goalkeeper Karl Darlow.
Forest centre back Kelvin Wilson was sent off in the 63rd minute for a second yellow after tackling Kike from behind.
Vossen added the second in the 79th minute, finishing off a fine move with a thumping finish, and Leadbitter completed the scoring in the 87th minute from the spot after Patrick Bamford was tripped by Darlow in the box.
Middlesbrough: Konstantopoulos, Nsue, Omeruo, Gibson, Friend, Clayton, Leadbitter, Adomah, Tomlin (Reach 74), Bamford (Williams 90), Garcia (Vossen 68). Unused substitutes: Whitehead, Veljkovic, Wildschut, Mejias.
Nottingham Forest: Darlow, Lichaj, Mancienne, Wilson, Fox, Christopher Burke, Tesche, Osborn, Antonio (Veldwijk 81), Fryatt, Assombalonga (Vaughan 46). Unused substitutes: Lascelles, Hunt, Laing, Paterson, De Vries.
Referee: S Duncan (Tyne & Wear).