Duterte, 81, served as president from 2016 to 2022, before being charged with crimes against humanity for setting up, funding and arming death squads that targeted narcotics peddlers and users.
He was arrested and taken to The Hague in March 2025.
Dressed in a black suit and white shirt, Duterte did not speak in court, but sat quietly sipping water during the proceedings on Wednesday.
He stood and waved to family members in the public gallery.
"We're happy that he has finally appeared and that he looks OK. In fact, he looks like he has put on weight," said Llore Pasco, whose two sons were killed during Duterte's war on drugs, watching a television broadcast of the hearings from Manila.
In court, Presiding Judge Joanna Korner asked prosecutors to find ways to shorten the trial, set to begin in November, due to the defendant's age and health.
"It's important this trial goes ahead as quickly as possible," she said, pressing parties to limit the number of witnesses and focus on key evidence.
Duterte's lawyer, Peter Haynes, called disclosure by the prosecution "enormous and impossible" and said his team could never examine tens of thousands of pieces of evidence in time.
"If I had a younger and healthier client, I would request pushing back the trial by six months," he said.
Details of Duterte's condition were not discussed in open court Wednesday, but his lawyers have said he suffers from cognitive decline that affects his memory and daily functioning, and is not well enough.
In January, ICC judges said Duterte's health was good enough for him to follow proceedings and requested his presence in court, rather than by video link from the detention unit.
The warrant for Duterte says he was responsible for a widespread and systematic campaign that led to thousands of killings between 2011 and 2019, including his time as the mayor of the southern city of Davao.
ICC prosecutors have said as many as 30,000 people might have been killed in the crackdown, the details of which were reported in a Reuters investigation.
Police said 6200 suspects were killed during anti-drug operations.
Duterte said he instructed police to act in self-defence and told supporters he was ready to "rot in jail" if it meant ridding the Philippines of illicit drugs.
