Devotional service includes Austerity

In the SB 9.4 Maharaj Ambarishes Qualities are described.In SB 9.4.27 it is described how by the performance of devotional service Maharaaj Ambarish thus also practiced severe austerities.

Srila Prabhupada's purport to this verse is very wondeful

SB 9.4.26: The king of this planet, Mahārāja Ambarīṣa, thus performed devotional service to the Lord and in this endeavor practiced severe austerity. Always satisfying the Supreme Personality of Godhead by his constitutional activities, he gradually gave up all material desires."

Srila Prabhupada's purport:

"Severe austerities in the practice of devotional service are of many varieties. For example, in worshiping the Deity in the temple there are certainly laborious activities. Śrī-vigrahārādhana-nitya-nānā śṛńgāra-tan-mandira-mārjanādau **. One must decorate the Deity, cleanse the temple, bring water from the Ganges and Yamunā, continue the routine work, perform ārati many times, prepare first-class food for the Deity, prepare dresses and so on. In this way, one must constantly be engaged in various activities, and the hard labor involved is certainly an austerity. Similarly, the hard labor involved in preaching, preparing literature, preaching to atheistic men and distributing literature door to door is of course an austerity (tapo-yuktena). Tapo divyaḿ putrakā. Such austerity is necessary. Yena sattvaḿ śuddhyet. By such austerity in devotional service, one is purified of material existence (kāmān śanair jahau). Indeed, such austerity leads one to the constitutional position of devotional service. In this way one can give up material desires, and as soon as one is freed from material desires, he is free from the repetition of birth and death, old age and disease."

Devotional service is about pleasing Lord Krishna.Performance of devotional service involves austeirities. If on just performs devotional service properly then he would also be performing austerities - activities which are pleasing to Krishna. Thus a devotee engaged nicely in devotional service does not have to separately perform austerities.