Olympic champion Bobby Finke of the United States won the men's 800m freestyle at the World Championships on Tuesday with an electric late burst to finish ahead of Germany's Florian Wellbrock and Ukrainian Mykhailo Romanchuk.

The win for Finke was a boost for the U.S team after 100m free Olympic champion and reigning world champion Caeleb Dressel pulled out of the semi-finals in that event for medical reasons.

Finke, who sat in fourth place for much of the race and was still behind three swimmers at the final turn, posted a personal best time of 7:39.36.

As he did in Tokyo, Finke timed his race to perfection, delivering a blistering final 50m, in 25.93 seconds to move ahead of his European rivals.

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"I knew I just had to just stick with them and stay with what I am good at the finish. I knew the 50 length was going to hurt but it was worth it," said Finke.

The loss of Dressel means he will miss an anticipated showdown with Romanian teenage talent David Popovici, who won the 200m free on Monday.

China's Yang Junxuan won the women's 200m freestyle in a time of 1:54.92 beating Australia's highly rated 18-year-old Mollie O'Callaghan, who took silver 0.30 behind.

The Chinese had two medals to celebrate with Muhan Tang taking bronze ahead of Britain's Freya Anderson.