SINGAPORE (Reuters) - Bitcoin sank to a four-month low on Friday and was on course for its worst weekly performance in a year, hurt by uncertainty over whether Joe Biden will remain the Democrats' U.S. presidential candidate and by worries about potential increases in crypto supply.

Bitcoin prices fell 5% to $55,366, their lowest since February end on Friday, and was down 10% for the week. Ether slid 8% to $2,891, a one and half month low.

Bitcoin had a strong start to the year after the launch of exchange-traded funds in the U.S., propelling it to a record $73,803.25 in mid-March. However, bitcoin has lost more than 21% since then.

Investors are also fretting about the possibility of Biden being replaced as the Democrats' presidential nominee by someone who is less pro-crypto, market participants said, after his poor performance in the first debate against Donald Trump.

Analysts also pointed to reports that Mt. Gox, the world's leading exchange for cryptocurrencies before it went defunct in 2014, is repaying its creditors. That has sparked concern that bitcoin is likely to face further downward pressure if those creditors offload their tokens.

(Reporting by Ankur Banerjee in Singapore; Editing by Edwina Gibbs)