Are you doing genomic DNA PCR or RT-PCR? For genomic PCR, adding the cell directly to the PCR mix should work. Initial denaturation for 5 min should be OK. Longer denaturation may shorten the life of the polymerase.
If you are doing qRT-PCR, try use Ambion® Single Cell-to-CT™ Kit.
Retired academic researcher. Mention of a specific product does not imply my endorsement of the product. No conflict of interest or guarantee to work on the advice given. Do as I say, not as I do. Not liable to the loss of your valuable samples.