Viagogo, the secondary-ticketing company, has clinched $15 million in new investment from two retired sports superstars at the same time as it has extended its current ticketing deal with Madonna.

The $15 million is from Stefanie Graf Ventures and Agassi Enterprises, which respectively belong to former tennis champions Steffi Graf and her husband Andre Agassi.

The two companies are investment vehicles focusing on lifestyle and sports ventures for Graf and Agassi and their stake boosts Viagogo's total external investment to $70 million so far.

Viagogo, based in London, will use the cash injection to expand globally, secure more celebrity and big-name deals, and enhance the service's technological infrastructure.

Graf and Agassi will be on Viagogo's board of directors, joining existing investors who include Lord Jacob Rothschild, of the banking dynasty, and Brent Hoberman, co-founder of e-commerce pioneer

Eric Baker, Viagogo's CEO/founder, tells "Currently, our two biggest markets are the U.K. and Germany. Expect announcements of expansion into other European markets and outside Europe."

He adds: "We also want to make new deals with more high-profile names, and plan to build on our technology and by making our Web site's backend system more robust to help our partners with the pricing, distribution and marketing of tickets."

The new investment comes just as Madonna has agreed to lengthen the arrangement to make Viagogo her official premium ticket seller and secondary-ticketing partner in the U.K. and Europe for her Sticky & Sweet Tour.

The original deal, groundbreaking at the time because of the involvement of a major star in the secondary market, started last May. But Madonna recently moved to extend the tour's U.K. and European leg this summer, starting with London's O2 Arena on July 4.

Viagogo has similar official arrangements with Andrew Lloyd Webber's the Really Useful Group, the international theater/entertainment production company, and some of Europe's biggest soccer teams like Chelsea Football Club.

It recently appointed Alex Zivoder, the ex-SVP of online travel agency Expedia Europe, to the new role of European managing director.

Baker admits that Viagogo is not profitable yet, but says it is growing two to three-fold every year and, he claims, at a faster rate than StubHub, the secondary-ticketing company he sold to eBay in 2007 for $307 million. "We're confident Viagogo will become profitable because we've done it before," he adds.