The fortune of Amazon.com founder Jeff Bezos surged by as much as US$6.6bn in extended trading on Thursday after the retailer reported third quarter results that topped Wall Street estimates.

If the gains hold through Friday, Bezos probably will overtake Microsoft co-founder Bill Gates atop the Bloomberg Billionaires Index, a daily ranking of the world’s 500 richest people. Amazon climbed 8.5% to $1 055.01 at 4.30pm in New York.

Gates ended Thursday with a net worth of $87.9bn, about $4.5bn ahead of Bezos. Gates has been the world’s richest person since 2013, though he briefly slipped behind Bezos during intraday trading on 27 July. Gates ended that day on top.

Bezos, 53, has added $18.1bn to his fortune this year, an increase of 28%. Gates’s net worth rose by $5.6bn, or 6.7%, even after he donated $4.6bn of Microsoft stock to the Bill & Melinda Gates Foundation in August.

Gates, 61, has given away almost 700m Microsoft shares and $2.9bn of cash and other assets since 1996, according to an analysis of his publicly disclosed giving. He would be worth about $150bn if he hadn’t made the donations. — Reported by Tom Metcalf, (c) 2017 Bloomberg LP