The three-day passes, costing $498, are for the Padang Grandstand and will offer spectators a view of the St Andrew's Road straight, in front of historic City Hall.
A check on the official ticketing website, www.singaporegp.sg, at various times of the day yielded no problems.
When 70,000 tickets for the public were released on Feb 14, keen interest led to a ticketing system meltdown. Frustrated fans were unable to get tickets despite queuing or waiting by computers for hours.
To avoid a repeat, the agent, OmniTicket, made various improvements to the system.
Tickets can also be purchased at all Singapore Post outlets, Forum Shopping Centre, Marina Square Shopping Centre and the Singapore Visitors Centre.
Fans can call 6738-6738.
Race organiser Singapore GP has declined to provide figures on how many tickets have been sold.
The Republic will host Formula One's first night race on a street circuit in the Marina Bay area on Sept 26-28.
Singapore Pools will, from today, start taking bets for Sunday's season-opening Australian GP.