Philippine President Rodrigo Duterte on Saturday said he will retire from politics after the end of his term, withdrawing his candidacy for vice president in next year's elections.

Duterte announced the surprise decision after accompanying his former longtime aide, Sen. Bong Go, who instead filed his own candidacy for the vice presidency at a Commission on Elections center.

Philippine presidents are limited by the constitution to a single six-year term and opponents had said they would question the legality of Duterte’s announced vice presidential run before the Supreme Court.

Duterte took office in 2016 and launched a crackdown on illegal drugs that left more than 6,000 mostly petty suspects dead and alarmed Western governments and human rights groups. The International Criminal Court is investigating the killings.

Political observers had long suspected Duterte could spring a surprise, such as a presidential run by his daughter, Sara Duterte-Carpio, next year.

Duterte-Carpio, who replaced her father as mayor of Davao, said last month she was not running for higher office next year because she and her father had agreed only one of them would run for national office in 2022.

The older Duterte's decision not to join the race next year would clear her way.

"This allows Sara Duterte to run," said Antonio La Vina, professor of law and politics at the Ateneo de Manila University. But La Vina said he could not rule out the possibility the firebrand leader could have a change of heart and be Go's substitute.

Candidates have until Friday to register, but withdrawals and substitutions are allowed until Nov. 15, leaving scope for last-minute changes of heart, like the 11th-hour entry of Duterte for the 2016 election, which he won by a huge margin.

Last month, Duterte's ally-turned-foe boxing star Manny Pacquiao announced his retirement from sports to run for the Philippine presidency.