On Sunday night the bowler was sent in as nightwatchman, a role he also fulfilled for New South Wales last season, with Glamorgan 4-133 chasing 283 to beat Somerset.
Five hours and 231 balls later Hadley hit the winning runs to bring up his maiden half-century as the Welsh county won by two wickets on Monday afternoon (local time).
It's a miracle," said Hadley.
""It happened in blocks. I was just hoping to chip away at a few to help the boys. I played and missed a few, luckily not good enough to nick it and Sean (Dickson) started really well."
Two not out overnight Hadley, who was dropped at second slip either side of lunch, added 114 with Dickson (76), then scored the bulk of the 37 runs added after the South African was dismissed.
The win moved Glamorgan into mid-table and kept Somerset off the top, a spot which is occupied by Fergus O'Neill's defending champs Nottinghamshire after a draw with Surrey, winners the previous three seasons.
England opener Ben Duckett scored his first century in 11 months, and went on to turn it into a double, finishing 203 not out as Notts batted throughout the final day, making 4-414.
Duckett turned his back on the Indian Premier League to work on his red ball game after a poor Ashes series and it has paid off with 503 runs so far this summer. He looks sure to keep his place for the New Zealand series with Zak Crawley, who has been in poor form, set to be dropped.
Sussex beat Jake Weatherald's Leicestershire by seven wickets, chasing down 131 to win despite a golden duck for Daniel Hughes. Beau Webster's Warwickshire thrashed Yorkshire by 377 to move into second.
In Division Two Nathan McSweeney's 206-minute 62 was not enough for Darren Lehmann-coached Northamptonshire, who lost to Caleb Jewell's Derbyshire by an innings and 113 runs.
Northants also had an Aussie nightwatchman, but Harry Conway made 0, though he did hold out for half-an-hour as the visitors tried to bat out the final day.
Gloucestershire captain Cameron Bancroft was just unable to engineer a win over Kent, who held on for a two-wicket win to leave Glos bottom.
