I would say that Hardy voluntarily laid the emphasis on ambiguity. Even the title of the chapter: The Seduction or Rape stresses ambiguity. Hardy wanted for the reader to make his own opinion. Moreover, he wasn't a manichean author. All his characters are stained by flaws, but also have some good in them. Somehow, even Alec can't be judged as an all evil character (though I DO hate him, but I try to put reason ahead for once ). The night where the turning point happened, he had just rescued Tess from the other girls, after the feast.
But, on the other hand, the name of the wood, the Chase is really symbolical. Alec is pursuing Tess and gives her no rest. He misleads her and is her downfall. Tess was asleep and very tired, even if she protested she may not have had enough strength to stop him. Not to mention she knew nothing of men and had told Alec several times that she didn't love him.
Yet, later in the novel, Tess aludes to it as a weakness of her own.
The fact that nothing is shown of the scene in the book really shows the grey vision of Hardy (or so I believe). Anyway, Tess is the embodiement of innocence and martyrdom all along the novel. This can't be contested. I think she was both raped and seduced, perhaps a bit more raped than truly seduced because Tess is also the kind of character who will always feel guilty about things she isn't responsible for.
This might help: Rape or Seduction?