Wellington Phoenix's looming A-League semifinal is heading for a sellout. A capacity crowd of 34,500 looks a formality.

Sales have quickly surpassed the 24,000 who turned up to watch the Phoenix beat the Perth Glory in a penalty shootout semifinal last week.

"We're already close to capacity at Westpac Stadium after just five days of ticket sales. The game is certain to be a sell-out and the remaining tickets won't last long," said Phoenix chief executive Tony Pignata.