Sir,
According to Section 51 (1) of CGST Act, 2017 "Notwithstanding anything to the contrary contained in this Act, the Government may mandate,––
(a) a department or establishment of the Central Government or State Government; or
(b) local authority; or
(c) Governmental agencies; or
(d) such persons or category of persons as may be notified by the Government on the recommendations of the Council,
(hereafter in this section referred to as “the deductor”), to deduct tax at the rate of one per cent. from the payment made or credited to the supplier (hereafter in this section referred to as “the deductee”) of taxable goods or services or both, where the total value of such supply, under a contract, exceeds two lakh and fifty thousand rupees:
Provided that no deduction shall be made if the location of the supplier and the place of supply is in a State or Union territory which is different from the State or as the case may be, Union territory of registration of the recipient."
Are you falling under any of categories enumerated at (a) (b) (c) and (d) above? Then only you have to deduct TDS from the amount payable to the service provider.
You say the service provider is unregistered. In that case you have to pay the tax under reverse charge mechanism under Section 9 (3) or 9(4) of CGST Act, 2017. This is my opinion.