I recently flew SGN-HKG-SFO. I was charged the full 30,000 miles but only got an upgrade from SGN-HKG. United refunded 20,000 miles since I didn't clear the HKG-SFO leg but I had to write to them to get the refund. The system didn't automatically refund. You may have to pay more than 10,000 miles though since SIN-NRT is farther than SGN-HKG.