* Posts 1.8 bln Y operating loss vs 5.2 bln Y profit expected
* Books 28.4 bln yen unrealised loss on its Lyft stake
* Steps up investment in new wireless service due for Oct launch (Adds background on Lyft, wireless service)
TOKYO, Aug 8 (Reuters) - Japan's Rakuten Inc reported an unexpected quarterly loss on Thursday, hit by the depressed value of its investment in ride-hailing firm Lyft Inc and heavy spending on a new wireless service.
The company said it booked a 1.8 billion yen ($17 million) operating loss in the April-June quarter compared with a 61.6 billion yen profit in the same period a year earlier. The market had expected a 5.2 billion yen profit, according to the average of five analysts compiled by Refinitiv.
Rakuten, whose billionaire founder and Chief Executive Hiroshi Mikitani has a seat of Lyft's board, recorded a 28.4 billion yen unrealised loss on its stake in the ride-hailing firm for April-June, as it had warned in July. It recorded a 110 billion yen gain in the previous quarter.
The results come a day after SoftBank Group Corp posted an unrealised loss on its stake in Lyft rival Uber for the April-June quarter. The two money losing ride-hailing firms are locked in a cash-burning battle for dominance in the U.S. market.
Rakuten cited an operating loss in its mobile services business, as it steps up investment in a new wireless service that is due to launch in October. The move will make it Japan's No.4 mobile carrier. ($1 = 106.0200 yen) (Reporting by Sam Nussey; Editing by Darren Schuettler and Muralikumar Anantharaman)