Tournament Director Terry Burwell said: "The Powergen Cup has already proved a hit with increased attendances on previous years in the Pool stages.
"Now fans of all four teams and rugby supporters in general are looking forward to a unique double-header at one of the world's best stadiums. It promises to be a great day out."
Steve Lewis, Welsh Rugby Union Chief Executive, is delighted with the early sales, saying: "To have sold over 20,000 tickets already is fantastic news for everyone involved. With such great clubs as London Wasps, Bath Rugby, Leicester Tigers and the Scarlets, there will be a truly fantastic festival of rugby on show.
"While Cardiff is the venue, the three English clubs will be bringing large contingents of supporters with them so it is important from a Welsh point of view that all of Wales get behind the Scarlets to make sure they benefit from a 'home' advantage in their match against Bath.
"With a Heineken Cup place on offer for an English winner, the Scarlets will have a tough challenge on their hands but this competition has certainly brought out the best in them so far this season."
Simon Hoskins, Senior Marketing Manager at Premier Rugby, confirmed that the Guinness Premiership clubs are committed to the new-look cup, which culminates in a final at Twickenham on Sunday, April 9.
"Holding the semi-finals at the Millennium Stadium makes it a really big occasion for both English and Welsh fans and a chance for supporters to watch two exciting matches for the price of one," said Hoskins.
"The incentive of automatic qualification for the Heineken Cup for an English winner of the Powergen Cup means that all our clubs are taking this competition very seriously.
"The total crowd for last year's two semi-finals was only 16,000 so to have already exceeded that number at this early stage is fantastic news."
Tickets, which includes both matches, cost £18 and £28 for adults and £9 or £14 for under-16s and are available from www.ticketmaster.co.uk or 08705 582 582. Powergen Cup semi-finals, Millennium Stadium, Cardiff, Saturday March 4. London Wasps v Leicester Tigers, 2.30pm, live on BBC1. Bath Rugby v Llanelli Scarlets, 5.15pm, live on S4C.