Kiwi all-rounder Corey Anderson was named man of the match for his 75 and two for 18 double in New Zealand's 98-run win, but the 24-year-old passed the credit to the two veterans in the side.
McCullum, he said, erased any nerves in the side when he blasted a rapid 65 in a quickfire 111-run opening partnership with Martin Guptill to set up New Zealand's impressive 331-6 in their 50 overs.
"We had a had a few nerves before the game, being the first World Cup game, but they they settled pretty quickly when Brendon and Guppy (Guptill) went about their work," Anderson said.
"We all know Baz (McCullum) as being someone that in a big event he really turns it on."
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After an impressive build up to cricket's showpiece ODI tournament the New Zealanders have been installed among the favourites, but Anderson said McCullum made sure the players did not succumb to public pressure.
Sri Lanka were on the verge of setting up an interesting finish to the match as they reached 124-1 in the 22nd over before Trent Boult and Vettori claimed three premier wickets for five runs in the space of 12 balls.
After Boult dismissed opener Lahiru Thirimanne for 65, Jayawardene strode to the middle and Vettori was immediately brought back into the attack and had the veteran caught behind without scoring.
"He's just so good, it's pretty easy to sum him up. He's played a lot of cricket and he knows how to get guys out.
"The two for 34 he got put them on the back foot and they had to try and take apart the other guys and that's pretty hard to do.