They prevented me in the day of my calamity - They took advantage of the time in which I was least able to make head against them, and their attack was sudden and powerful. I should have been overthrown, but the Lord was my stay. He had been nearly exhausted by the fatigue of the day, when the giant availed himself of this advantage.
They prevented me - They anticipated me, or went before me. See the note at Psalm 18:5. The idea here is that his enemies came before him, or intercepted his way. They were in his path, ready to destroy him.
In the day of my calamity - In the day to which I now look back as the time of my special trial.
But the Lord was my stay - My support, or prop. That is, the Lord upheld me, and kept me from falling.