The Scot's grouping had already been delayed by rain.
"I was just sitting there with my wife having a yoghurt and then I looked at Twitter and I thought 'I'm meant to tee off in five minutes'," he said.
"Nobody came and told us. So I got kind of lucky that they made another 10 minute delay before we could go."
Knox recorded a second consecutive even par round of 70 at Baltusrol to ensure he would make the cut of two over par.
But he was left frustrated by a round that left him nine strokes behind joint leaders Robert Streb and Jimmy Walker of the United States.
"We got very lucky with the fact that we were on the first tee and they had a little delay and then, when we went back, it had stopped raining," he told BBC Sport.
"I've played better than I've scored. I made a couple of soft bogies today - middle of the fairway, eight-iron - and you just can't do that. Not when there's enough tricky holes."
Knox needs to win or finish runner-up to force his way into the nine automatic places in the European team for the forthcoming Ryder Cup.
Anything less and the world number 26 would be left relying on captain Darren Clarke choosing him among his wildcards.
"I feel that I've played well, but I just haven't got the putter going and haven't been able to create any momentum," added Knox.
"All my birdies have just been really good shots, which is good, but in order for me to get something going, I have to hole some putts and I just haven't been able to do that yet.
"For whatever reason, I have just not felt all that comfortable, so hopefully the weekend will be different.
"I know how to putt. I've just got to let it happen."
Russell Knox was thankful for a rain delay that ensured he did not miss his tee-off time in the second round of the US PGA Championship.