* Putting problems persist for world number one * Spieth plays 28 holes on Saturday (Updates at end of play) By Patrick Johnston SINGAPORE, Jan 30 (Reuters) - World number one Jordan Spieth struggled again with his putting as he ended a long 28-hole Saturday three shots off the pace at the Singapore Open. The six-under-total left him tied fifth, three shots behind overnight leader Song Young-han of South Korea who was level through 13 holes of his third round before play was called off. Spieth, the headline act of the Asian and Japan Golf Tour's season opening event, had begun Saturday five adrift of Song but felt the gap should have been closed further. More...