Bruised, aching and exhausted - yes - but undefeated, the Black Caps skipper had reached 281 when stumps were pulled last night and was in sight of Martin Crowe's long-standing New Zealand record individual score of 299.
A restless nation will collectively hold its breath, stand on one leg, cross their fingers or do whatever they think might help the 32-year-old right-hander find another 19 runs when the second test against India ends today.
No doubt those extra runs will lift the innings from something merely extraordinary to a grander place in our folklore. But whether he gets the runs or not, his marathon, almost surreal, 12-hour stint at the crease has captured our attention.
Speaking after the day's play, McCullum said he was ''a little bit tired''.
''The last hour was a bit of a daze, in all honesty.''
It has to rate as one of the greatest innings played by a New Zealand batsman. It is certainly the longest.
So far McCullum has spent 726 minutes at the crease. That eclipses the 704 minutes Glenn Turner spent reaching 259 against the West Indies, in Georgetown, in 1971-72.
Consider, too, that McCullum came to the wicket when New Zealand was 52 for three. The situation only got worse. By the time BJ Watling joined him in the middle, the Black Caps were 94 for five.
Defeat seemed certain. But the pair dragged New Zealand back into the match with a world record sixth-wicket stand of 352. Watling himself batted for a remarkable eight and a-half hours, faced 367 balls and scored 124.
McCullum appeared so focused on saving the test he barely managed a smile when he became the first New Zealand batsman to score 200 in consecutive tests.
His innings of 224 earlier this month helped set up New Zealand's 40-run win against India in the first test.
He also joined Stephen Fleming as the only New Zealand batsman to have scored three test double 100s - all that while leading his country to three consecutive test wins.
Not bad for a player who at the start of the summer had not scored a test 100 for three years and whose role as captain and even his place in the side was sometimes questioned. Any lingering doubt was vanquished with this remarkable knock.
