Stefanos Kasselakis, a 35-year-old Greek former ship investor and Goldman Sachs trader without any previous political experience or track record, is set to become the leader of the country’s main opposition, centre-left Syriza party.

According to results covering 98% of the vote, Kasselakis won 45% of the vote against 36% scored by Syriza insider and former labour minister Effee Achtsioglou.

Having beaten off a field of lesser rivals, Kasselakis and Achtsioglou, 38, will face off in a second-round vote between them on 24 September.

Kasselakis earned a small fortune last year through opportunistic bulker and tanker asset plays of his shipping investment vehicles SwiftBulk, SwiftTanker and Tiptree Marine.

He entered the Syriza leadership race as a total outsider.

A Miami resident who was not even a party member, he publicly declared his candidacy on 29 August.

Being a newcomer to Syriza, he lacked the party-political credentials usually required to promote him to the rank and file.

As the campaign progressed, however, the telegenic and social-media-savvy Kasselakis attracted new members to the party.

Furthermore, he successfully coalesced with Syriza MPs and party barons who lost their seats or saw their internal party standing diminish after a string of devastating political defeats.

The formerly radical and now more moderate Syriza party held power in Greece between 2015 and 2019 but was trounced in two consecutive parliamentary elections since.

Its latest disastrous result in June sent the party in a tailspin, with former leader and then prime minister Alexis Tsipras quitting active politics and opening the field to lesser-known hopefuls.

Kasselakis’s campaign combined populist anti-elite rhetoric with traditional progressive themes, such as the separation of church and state, the defence of gay and LGBT rights, as well as the abolition of mandatory military service.

“The first step was done for the country to have a progressive government soon,” Kasselakis said early on Monday after his election victory.