Cuba's twice Olympic champion Arlen Lopez Cardona launched his bid for a record-equalling third gold medal with a unanimous win at the Paris Games on Tuesday, while Australia and India suffered three defeats each.
Lopez Cardona, middleweight champion at the 2016 Rio Games and light heavyweight champion five years later in Tokyo, is looking to become only the fourth boxer to win three Olympic golds alongside Teofilo Stevenson, Felix Savon and Laszlo Papp.
The 31-year-old had a slow start but bossed the last two rounds to prevail 5-0 against Turkey's Kaan Aykutsun and set up a clash against Uzbek Turabek Khabibullaev.
"I think that my corner knew how to do the job that it has been able to do for years," Lopez Cardona said.
"And I, as a boxer, and as an experienced one, knew how to do it as well. We helped each other. And we were able to get the second and third round."
Olympic silver medallist Oleksandr Khyzhniak progressed to the light heavyweight quarterfinals with a 4-0 win over Hungary's Pylyp Akilov, though it was not all smooth sailing for the Ukrainian as he struggled in the first round.
In the opening flyweight bout of the day, France's Billal Bennama did well to beat American Roscoe Hill by getting in close and letting fly with quick flurries of punches to claim a 3-2 Round of 16 win.
Tokyo Olympics bronze medallist and twice world champion Huang Hsiao-wen of Taiwan was eliminated by Bulgarian Stanimira Petrova, who advanced to the quarterfinals of the women's bantamweight division.
The win sees Petrova reach the quarterfinals for the first time in her third appearance at the Olympics.
"Every time I want more and more from my fights and we're already thinking about the next bout," Petrova said.
"After losing in Tokyo, I died. I really think I died. I wanted to stop boxing, but I love boxing and I will continue until the end."
Australia and India had days to forget.
Australia's Yusuf Chothia lost 4-1 to Spain's Rafael Lozano in a flyweight bout, before light heavyweight Callum Peters and bantamweight Tiana Echegaray were beaten.
India's Asian Games and Commonwealth Games champion Amit Panghal lost 4-1 to Zambian Patrick Chinyemba, while featherweight Jaismine Lamboria and bantamweight Preeti Pawar were eliminated.
Lamboria was beaten by Tokyo Olympics silver medallist Nesthy Petecio of the Philippines.