The 27-year-old Dutchman, a rider for the Alecto Cycling team, was placed in an artificial coma for 48 hours and spent the following three weeks in critical condition.
On Thursday night his situation deteriorated and his death was announced on Friday morning.
"Unfortunately we have to confirm with great sorrow that Robbert died last night due to complications after his cardiac arrest," read a statement from the Alecto team.
"Thanks for everything Robbert, we will never forget you."
De Greef was on the first cobblestone sector of the Omloop van de Braakman just 15 kilometres into the race when he suffered the cardiac arrest. The emergency services attended to him at the roadside, performing extensive CPR, before he was transferred to hospital in Antwerp. The race was subsequently cancelled.
Just over a week on, the Alecto team said "there is some hope again", with his condition stable, if still critical. Last weekend, as they lined up for the Arno Wallaard memorial - a race in memory of another rider who died of heart issues - they said that when his health improved he would be eligible for a heart transplant.
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On Friday, however, they had to communicate the worst possible news. According to Dutch newspapers, De Greef suffered a brain haemorrhage during the night, which led to his death.
After starting his career at Continental teams in Holland, De Greef rode for the Roompot-Nederlandse Loterij Pro Continental team last season. After the team's merger with Veranda's Willems-Crelan, he moved back to Continental level with Alecto. He had enjoyed a strong start to the 2019 season, with second place at the 1.HC Ronde van Drenthe, finishing three seconds behind the winner Pim Ligthart.
Cyclingnews extends its sincere condolences to De Greef's family, friends, and teammates.
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