"It's marginal at the moment," Smith said. "If it was anything else I would just play no worries, but with my hammies and my history I have to act with caution.
"I've been getting fitter every week. I don't have the pace anymore, I'm not as quick as I used to be but I try to make up for that with smarts."
Smith's big game experience would be a huge asset to a young Birkenhead side. He won the Chatham Cup as a 17-year old with Napier City Rovers - partnering Shane Smeltz up front - has played for Toronto FC and Seattle Sounders in the MLS and Swedish outfit Ljungskile SK.
Birkenhead face a Central side re-upholstered by a number of recent Auckland City additions. ASB Premiership golden boot winner Emiliano Tade will lead the line while Korean import Dae Wook Kim and Japanese left back Takuya Iwata will add quality.
"We've played them already, when they didn't have their national league boys back and they beat us then at home, so we know the quality they are going to bring. But we're also quietly confident that we have the players to test them," Smith said.
"Last weekend there were periods where we were class and played Three Kings off the park, but then the week before we were very average for about 70 minutes against Bay Olympic. So it's about finding that consistency for us."
Other key third round matchups on Saturday include Lower Hutt City AFC hosting Miramar Rangers, while Waitakere City FC will host Melville United in a rematch of the 2013 semi-final.