Austin “Ammo” Williams spectacularly stopped a stubborn substitute late in the seventh round of their 10-round middleweight match Saturday in Las Vegas.

The undefeated Williams dropped Armel Mbumba-Yassa once apiece in the sixth and seventh rounds and won by seventh-round knockout in the co-feature before the 10-round main event between Conor Benn and Peter Dobson at The Cosmopolitan of Las Vegas. Referee Harvey Dock stopped their fight at 2:52 of the seventh round, immediately after Williams knocked Mbumba-Yassa flat on his back with a left hand.

Milwaukee’s Williams, a highly rated middleweight contender, improved to 16-0 and produced his 11th knockout. Germany’s Mbumba-Yassa lost for the first time as a pro (10-1, 7 KOs).

Mbumba-Yassa replaced Connor Coyle, Williams’ original opponent, on less than one week’s notice. Coyle (20-0, 9 KOs) suffered an elbow injury during training camp that forced him to withdraw from their scheduled 12-rounder late last week. Mbumba-Yassa entered the ring unbeaten, but he hadn’t faced an opponent on Williams’ level in any of his first 10 professional fights.

Mbumba-Yassa demonstrated toughness by lasting as long as he did, but Williams was the more skillful, powerful fighter throughout their bout.

After a very successful sixth round, Williams didn’t land many punches in the seventh round until he blasted Mbumba-Yassa with a left hand that knocked him flat on his back, beneath a bottom rope, with 13 seconds remaining in it. Dock halted the action soon thereafter, as Mbumba-Yassa attempted to get up.

Williams went on the offensive during the opening minute of the sixth round, when he unloaded hard head and body shots. A right to Mbumba-Yassa’s body and subsequent head shots sent Mbumba-Yassa to the canvas with 1:35 to go in the sixth round.

Mbumba-Yassa beat Dock’s count, and then moved and held his way to the bell to end the sixth round.

Williams buzzed Mbumba-Yassa when he landed a straight left while Mbumba-Yassa was backed against the ropes with 30 seconds to go in the fifth round. Mbumba-Yassa initiated a clinch a few seconds later to stop Mbumba-Yassa from punching.

Williams’ left landed in an exchange 30 seconds into the fifth round.

Williams’ left uppercut knocked Mbumba-Yassa off balance with about a minute remaining in the fourth round. An overhand left by Williams backed Mbumba-Yassa into the ropes about 30 seconds into the fourth round.

Mbumba-Yassa blocked several of Williams’ punches and then connected with a right to the side of Williams’ head with just over 30 seconds on the clock in the third round.

A straight left by Williams landed cleanly a little less than a minute into the second round. Williams nailed Mbumba-Yassa with a left to his body that landed with just under a minute to go in the second round.

Williams took a patient approach during the first round, when he blocked several of the right hands Mbumba-Yassa threw at him from long range.

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.