The impressive performance from Ward was marred by the ending to the fight, with a succession of borderline low blows securing the stoppage after he had initially rocked the Russian with a powerful straight right to the head.
"Then I hurt him with a head shot and I just had to get the right shots in there to get it over with".
Ward had received a hotly disputed points victory in their first fight and there will be fresh debate after he retained his WBA "super", IBF and WBO belts.
Andre Ward and Sergey Kovalev met in one of the biggest fights of the year in November 2016. But as the time went by, Ward was adapting more and more, eventually turning this fight into his advantage.
The light heavyweight champ told Complex on June 12, "The only thing I've noticed about him [Kovalev] that's different since the fight ended is just the lack of mental toughness that this guy has".
Kovalev (30-2, 26 KOs), known for his formidable punching, came out charging in the opening rounds, but was left frustrated by the counter-punching of ward and frequent grabbing.
"He's a good fighter, and I have nothing but respect for him", he said.
Sergey has since deleted the video meme, but the backlash was swift from those who said that making it sound as if Ward - a devout Christian - dropped nonstop n-bombs was racist. That's why I want to face the best, to be the best. I think that would be a very interesting fight.
In the featherweight division, Tramaine Williams remained undefeated by stopping the more experienced Christopher Martin in two rounds, whilst Enriko Gogokhia out-pointed Jonathan Steele over six rounds at welterweight. "When I hit him he was acting like I hit him low".
'Kovalev also was probably up in the fight, at least by a little bit. Kovalev's been getting heat for a meme he posted to his social media recently, one that depicts a fake convo between him and Ward, where Andre repeatedly uses both the N- and F- words. And, if you choose to make a defense for Kovalev's plodding battle plan, there were definitely the low blows. And he's the best, but styles make fights. There were low blows.
Agnew, who had only previously lost to Sergey Kovalev and Samuel Clarkson, went into survival mode and somehow made it through the next two sessions only for the referee to spare him more punishment midway through the fourth.
"I can not explain", the 34-year-old said. I wanted to continue fighting. I dont feel like it affects me. There a lot of people who are convinced I won it.. Ward right to the body. Close round I give it to Kovalev.
After the fight, Ward gave Kovalev his credit and admitted he was the most hard opponent of his career.
Ward was more interested in looking ahead, arguing that he has a better case than unbeaten three-belt middleweight champ Gennady Golovkin for being the No. 1 pound-for-pound boxer.
Andre Ward's trainer Virgil Hunter believes the light heavyweight king could survive a jump to the top weight class and "outbox" Anthony Joshua. Who he fights next?