The Proteas clinched a comfortable victory in Johannesburg by bowling Pakistan all out to secure a winning lead of 211 runs.
Steyn, who took four wickets in the morning session, reached a career-best of 11-60 and has admitted that he performed "nicely".
"I've not been able to pick up wickets at the top in recent times but I'm glad I did it here," BBC Sport quotes Steyn as saying. "Things just kind of clicked, the ball came out nicely.
"There's not much thinking going on, your natural instinct takes over. I just got the ball in my hand and ran in and bowled."
Meanwhile, South Africa wicketkeeper AB de Villiers took his 11th catch to equal the world record for most dismissals in a match.