After breaking Brazilian hearts by beating Mariana Silva in the semi-finals, top seed Trstenjak defeated Agbegnenou by ippon less than two minutes into the final. Israel's Yarden Gerbi and Anicka van Emden of the Netherlands won bronze.
The Slovenian believes that her ability to stay focused and her sound tactical preparations were crucial in helping her overcome the odds, both against home favourite Marian Silva in the semis and against her much fancied French opponent in the gold medal bout.
"We made a good tactic with my coach,” she said. “The bout against Mariana Silva was very tough, because the full stadium was against me, but I was focused and finally I came out on top.
"Clarisse Agbegnenou is a very strong judoka but she made a mistake and I took advantage of it. I practised this move a lot, so today my training was the key to my success."
For her part, the French judoka admitted to mixed emotions, but acknowledged that coming away with a silver medal around her neck was still a fine achievement.
"This medal does not have the right colour, but it is still beautiful. I wanted to open France’s medal count with a nice gold medal but the silver medal is not bad either.
"For the moment there is disappointment. After the defeat, I can't be pleased, but I am still happy to have this medal."
Agbegnenou's silver did bring an end to France’s judo medal brought in Rio. The French contingent arrived at the Games as just one of three NOCs fielding a full team of 14 judokas, and were widely expected to feature prominently among the medals, but after the first three days of action had remained empty-handed.
Her compatriots will now be hoping that Agbegnenou can kick-start a medal rush and bring them closer to emulating their performance in London four years ago, when French judokas claimed a total of seven medals, a tally matched only by judo superpower Japan.
Later the same night, the focus shifted to the men's -81kg, in which Russia’s Khasan Khalmurzaev also made quick work of his opponent, the USA’s Travis Stevens. The American’s hopes of winning his country’s first ever judo gold were dashed as he was thrown to the mat for ippon with over two minutes of the bout remaining.
“I know this opponent and I know he is strong, so I did all I could do to win this gold medal,” said the Russian.
"I fought him before and he is very strong on the ground. You saw yourself that he won other matches on the ground. So I knew that I had to fight him upright.
"He did a very good turn but I realised what I had to do and how to defend.
"Many people believed in me; everything worked out and I am very happy. There is probably no limit to the joy I feel and I want to thank everyone who led me to this competition."
There was a silver lining for Stevens, who had failed to make the podium at Beijing 2008 and London 2012. It was third time lucky for Sergiu Toma too, who claimed one of the two bronze medals, competing under the flag of the United Arab Emirates, with a defeat of Italy’s Matteo Marconcini.
The other bronze in the -81kg category went to 2015 world champion, Takanori Nagase, who had previously suffered a surprise defeat to Toma in the quarter-finals, but bounced back to overcome Avtandil Tchrikishvili of Georgia. Nagase’s medal was Japan’s sixth judo bronze at these Games so far.