The tourists have already downed Pakistan 1-0 in the Test series but Smith said limited-overs cricket was a different proposition ahead of Thursday's opener.
"Pakistan's confidence will be high after their recent success in the Twenty20 tournament," he said.
"But obviously it's another challenge for their captain Shoaib Malik and we will try to put them under pressure."
Pakistan were narrowly beaten by arch-rivals India in the inaugural Twenty20 final in Johannesburg.
But Smith said South Africa were feeling fresh and rested after sealing their first series victory on the subcontinent since 2000 last week.
In another confidence boost, they beat Pakistan's Patron's XI by 176 runs in their only warm-up match here on Tuesday.
"We are pretty confident after a rush start to the tour and it was nice to have some good rest and we are going into the one-day series knowing we have the potential to do well," Smith told a press conference Wednesday.
"It is always important to have a good start," said Smith.
The only cloud hanging over the Proteas was a hamstring injury to all-rounder Vernon Philander, the skipper said.
"He hurt his hamstring and is struggling to walk so we are not sure about him," said Smith of Philander, who arrived during the first Test in Karachi as a replacement for paceman Morne Morkel.
Pakistan captain Malik said he was confident his team would shrug off the disappointment of losing the Test series.
"We need to put the Test series loss behind us and I know all the players are professional and know their responsibilities, so I am confident of a good show," said Malik.
Pakistan are unsure of Mohammad Asif's fitness after the fast bowler re-injured his elbow during last week's second Test in Lahore.
"Asif will bowl in the nets and only then will we take a final decision on him," said Malik.
However Pakistan will be bolstered by the return of all-rounder Shahid Afridi who sat out the Test series because they coincided with the Muslim holy month of Ramadan.
TEAMS:
SOUTH AFRICA: Graeme Smith (capt), Herschelle Gibbs, Jacques Kallis, A.B. de Villiers, Justin Kemp, Mark Boucher, Jean-Paul Duminy, Vernon Philander, Albie Morkel, Andre Nel, Makhaya Ntini, Shaun Pollock, Johan Botha, Charl Langeveldt
PAKISTAN: Shoaib Malik (capt), Salman Butt, Mohammad Hafeez, Imran Nazeer, Yasir Hameed, Younis Khan, Misbah-ul Haq, Shahid Afridi, Mohammad Yousuf, Kamran Akmal, Sohail Tanveer, Rao Iftikhar, Abdul Rehman, Mohammad Asif, Umar Gul.

Copyright AFP (Agence France-Presse), 2007