In addition to the excellent reasoning of the existing answer, I would like to add one detail.
Although not much more can be gleaned from the edition that is in circulation today, the 1891 first edition of Tess of the d'Urbervilles contained some details that were later removed. During the episode at the Chase, Alec actually forcibly drugs her in a very unambiguous act, in order to make sure she is asleep before coming back and subsequently taking advantage of her in her senseless state:
“Nights grow chilly in September. Let me see.” He went to the horse, took a druggist’s bottle from a parcel on the saddle, and after some trouble in opening it held it to her mouth unawares. Tess sputtered and coughed, and gasping, “It will go on my pretty frock!” swallowed as he poured, to prevent the catastrophe she feared.
“That’s it - now you’ll feel warmer,” said d’Urberville, as he restored the bottle to its place. “It is only a well-known cordial that my mother ordered me to bring for household purposes, and she won’t mind me using some of it medicinally. Now, my pretty, rest there; I shall soon be back again.”
In the revised and now definitive 1892 edition this was changed to
"Nights grow chilly in September. Let me see." He pulled off a light overcoat that he had worn, and put it round her tenderly. "That's it--now you'll feel warmer," he continued. "Now, my pretty, rest there; I shall soon be back again."
which practically transforms all connotations by replacing the object that he uses to 'warm' Tess with his "light overcoat" instead of the "druggist's bottle". Tess' sleep as a product of simple tiredness in the newer edition is subsequently a drastic change from the drugged sleep and consequently undeniably sinister intent of the action contained in the original edition.
Alec drugs her against her will which confirms that the sexual act between Alec and Tess was premeditated rape and definitely not consensual. Most likely this was removed from the final edition to censure the violence of Alec's actions to make it more socially suitable reading, but this reveals Hardy's original intention to cement in Victorian minds Alec as the violator of an actual, blatant crime and Tess, without any doubt, as the victim. The Ashgate Research Companion to Thomas Hardy Dr Rosemarie Morgan ed., which further details this first edition scene, cites (p. 106) that a contemporary law meant that
If a person by giving a woman liquor makes her intoxicated to such a degree as to be insensible, and then has connection with her, he may be convicted of rape
exacting precisely what Alec does. However, scrutiny of Tess' blamelessness and purity (as established in the subtitle of 'A Pure Woman') arose after Hardy, for whatever reason, decided to omit this intrinsic clue and soften the degree of threat of Alec's act, creating new ambiguity around what was unquestionably rape in the original edition.