Bruce Mitchell of South Africa (89.85) heads the list of fourth-innings averages, ahead of West Indies' Jeffrey Stollmeyer (86.33) and Australian Peter Burge (76.50).
Williamson's latest century was another addition to what is becoming a series of remarkable feats.
Yes, the innings came against Bangladesh, but the visitors held the balance of power in the test until Williamson strode out for the final time. His technique and temperament disarmed their dreams of a maiden test win against New Zealand.
It had echoes of Daniel Vettori's scruff-of-the-neck leadership at Chittagong in 2008 when New Zealand faced the daunting prospect of chasing 317 for victory. Vettori made 76 batting at No4, having earlier scored 55 not out and taken nine wickets for 133. He received support from Aaron Redmond (79) and Daniel Flynn (49) but by the time the skipper exited at 298 for six, the result was under control.
Arguably Williamson's best fourth innings contribution was his first, 102 not out, against South Africa in 2012 at the Basin Reserve.
Neither Dale Steyn, Vernon Philander nor Morne Morkel was capable of luring him into a dismissal across 327 minutes and 228 balls.
The knock included a Steyn delivery which cracked his protective box. For a split-second, Williamson must have felt like his world was imploding, yet he endured a further 181 balls to survive the day.
Legend has it Steyn ventured across for a post-test chat and signed the fragmented shell. Williamson has moments of vulnerability like any other batsman, but they occur less often, particularly in the final dig when the pitch is at its most worn and scoring becomes more difficult.
His batting is probably more pragmatic than romantic. His drives are often checked without exaggerating the pose and his attacking shots on the legside are rarely aerial, but such conservatism does not stop him using his feet to spin.
The way he remains unperturbed in the face of menacing bowling and fan expectations suggest a sportsman of rare composure is on the rise.
