The fight ended at the 1:22 mark, when Ponzinibbio (25-3) knocked Nelson out cold with a left hand along the fence. It is the biggest win of Ponzinibbio's career and headlined the event at SSE Hydro Arena in Glasgow, Scotland.
Originally from Argentina, now fighting out of South Florida, Ponzinibbio stunned Nelson (16-4-1) with a right hand in the center of the cage early. Nelson backed to the fence and tried to cover up, but Ponzinibbio followed with a clean jab that ended the fight.
The result was an upset and spoils the growing hype behind Nelson, who trains alongside UFC champion Conor McGregor and had won three of his previous four.
"Gunnar Nelson is a tough guy. I really like his style," Ponzinibbio said. "This was a very big test for me.
"I am here to face the best fighters in the world. I am the best striker in this division. I am the next champion of this division. Come on UFC, I am ready. Give me who you like. I am ready for everything and everyone."
Even though the fight didn't last long, it featured several significant exchanges. Nelson opened things up with an uppercut, left-hand combination that got Ponzinibbio's attention. He also darted in with a lead left that caught Ponzinibbio flush.
Ponzinibbio displayed his power in the first punch he landed, however -- that right hand out in the open. In terms of total strikes, the two punches that hurt and then knocked out Nelson were the only ones Ponzinibbio was credited for.
The 30-year-old Ponzinibbio extended his win streak to five with the victory. Nelson, who had been campaigning for a highly ranked opponent prior to Sunday, is now 7-3 in the UFC.
Surging Calvillo edges Calderwood
Strawweight Cynthia Calvillo continues to impress in the UFC, as she picked up her third Octagon win in the past four months.
Calvillo (6-0) edged Joanne Calderwood (11-3) via unanimous decision (30-27, 30-27, 29-28) in a tactical three-round affair. Calvillo nearly finished Calderwood with a rear-naked choke in the closing seconds of the fight.
Calderwood, who is from Glasgow, had some success defending Calvillo's takedown and scored frequently with inside low kicks, but it was Calvillo who secured the key moments. She took Calderwood down in the first and third rounds and threatened with chokes and armbars. She also bloodied Calderwood's nose with a left hook, right hand combination in the second.
Fighting out of Team Alpha Male in Sacramento, Calvillo made her UFC debut in March and has already racked up wins over Calderwood, Pearl Gonzalez and Amanda Cooper. Calderwood suffered consecutive losses for the first time in her career.
In memory of late father, Paul Felder scores huge KO
Felder, of Philadelphia, lost his father to cancer just one week before starting his camp for Ray in Milwaukee. The 32-year-old wore a small capsule filled with his father's ashes around his neck to the Octagon.
"About a week before training camp started, my father died of pancreatic cancer," Felder said. "I was there with him in hospice, and he's here in this chain with me tonight."
Ray, who is from Scotland and had the crowd behind him, unsuccessfully worked for takedowns early. The two ended up jockeying for position in the clinch, and Felder landed a beautiful knee up the middle that dropped Ray to his back. He finished the fight with hard elbows out of Ray's guard.
Felder has quietly won four of his past five contests, recording finishes in three of those. He is 6-3 overall inside the Octagon.
Rountree puts Craig away in opening round in enemy territory
Fighting out of Las Vegas, Rountree held an obvious advantage over Craig on the feet, and he dictated the pace and style of the fight from start to finish. Craig, of Scotland, looked timid throughout and never came close to scoring a takedown.
The finish came at the very end of the round, after Rountree dropped Craig with a straight punch, which was set up by a high head kick.
A former contestant on The Ultimate Fighter reality series, Rountree got off to a shaky 0-2 start in the UFC but is 2-0 since with back-to-back first-round knockouts. Craig falls to 1-2 in the UFC.