Gandalf Vs. The Witch King

[Essex]

I've just noticed this in the book, when Gandalf has his chat with Denthor:

'Yet now under the Lord of Barad-dûr the most fell of all his captains is already master of your outer walls,' said Gandalf. 'King of Angmar long ago, Sorcerer, Ringwraith, Lord of the Nazgûl, a spear of terror in the hand of Sauron, shadow of despair.'
'Then, Mithrandir, you had a foe to match you,' said Denethor. 'For myself, I have long known who is the chief captain of the hosts of the Dark Tower. Is this all that you have returned to say? Or can it be that you have withdrawn because you are overmatched?'
Pippin trembled, fearing that Gandalf would be stung to sudden wrath, but his fear was needless. 'It might be so,' Gandalf answered softly.

Seems like Denethor thinks the WK is Gandalf's match, and Gandalf doesn't argue!