Last weekend's game against the All Blacks saw the Millennium Stadium host a record attendance in Wales for a game against New Zealand of 74,024 - the highest crowd figure of any of the 14 Autumn International fixtures across Europe to date.
"Ticket sales for the game against Japan have been very steady since the thrilling game against the All Blacks on Saturday. The Welsh fans obviously want to see more of the same and give this exciting team their backing," said Millennium Stadium general manager Paul Sergeant.
"We had a walk-up crowd of more than 8,000 on the night we played Romania and at our present rate of sales a repeat of that would take us well past the 50,000 mark. In order to accommodate any more than 43,000 we have to open the lower tier.
"We've already put extra stewards on stand-by for the night and we are urging everyone who wants to come to the game to buy their tickets before the day of the game. Not only will that mean they can buy them more cheaply - they are £8 and £4 before Friday and £10 and £5 on the day of the game - but it should also mean we can avoid having to delay the kick-off again."