(Removes reference to Monday in first paragraph; corrected third paragraph to show that the OpenX platform is software as a service, not open source)
LONDON, May 30 (Reuters) - SAP Ventures is leading a $20 million funding round in online ad exchange and server OpenX, an independent challenger to Google's (GOOG.O) DoubleClick, OpenX said.
SAP Ventures, German business software company SAP's (SAPG.DE) venture fund, will be joined by additional new investors AOL Ventures AOL.N, Mitsui Global Investment (8031.T) and Sumitomo's (8053.T) Presidio Ventures.
The series D round brings total investment in OpenX to $50 million, as investors bet the software-as-a-service platform will win a big chunk of a still relatively undeveloped online ad market.
"DoubleClick has been dominant for so long. It is a just a question of time until somebody comes along with a bigger, better mousetrap, and we very much hope OpenX will do that," Nino Marakovic, managing director of SAP Ventures, told Reuters.
The internet is expected to overtake newspapers to become the world's second-largest advertising medium after television in 2013, according to media buyer ZenithOptimedia (PUBP.PA), as ad spending catches up with time spent online.
Existing investors Accel Partners, Index Ventures and DAG Ventures also participated in the latest round of OpenX's funding.
"We are looking to grow pretty aggressively internationally," OpenX chief executive Tim Cadogan told Reuters. He said he hoped OpenX would be in China in the next 12 months, and that South Korea was also a top priority in the near term.
OpenX allows buyers and sellers to trade online advertising spots in real time and also serves up the ads to the websites.
In recent months it has displaced DoubleClick at Orange FTE.PA, and formed a partnership with leading Asian ad agency Dentsu (4324.T). (Editing by Dan Lalor)