Olympic champion Viktor Axelsen powered into the world number one spot on Wednesday after a walkover victory over fellow Dane Rasmus Gemke in the group stage of the World Tour Finals in Indonesia.
Axelsen, 27, took men's singles gold at this year's Tokyo Games and moves above Japan's Kento Momota as Gemke withdrew from their match injured.
Momota had been the world's leading men's player since September 2018 and also pulled out of his fixture, just one minute after it started.
"I am so sorry to my fans to end up like this," said Momota, who sustained a back injury in training earlier in the day.
Momota, who won a record 11 titles in 2019, and Gemke dropped out of the competition as their remaining matches were considered walkovers under the tournament regulations.
Axelsen is now favourite for the title and will meet unseeded Indian player Lakshya Sen in his next group match on Thursday.
The World Tour Finals, returning to Asia for the first time in 10 months, is the season-ending of three consecutive tournaments played in strict bio-secure conditions and without spectators.
Earlier, Japan's Akane Yamaguchi completed her women's group match in 34 minutes, defeating Singapore's Yeo Jia Min in straight sets 21-11, 21-14.
Thailand's top mixed doubles pair Dechapol Puavaranukroh and Sapsiree Taerattanachai beat home favourites Praveen Jordan and Melati Daeva Oktavianti 21-14, 10-21, 21-11.
"Today's matches were tight. We were under pressure and almost found it difficult to find the game rhythm," said Sapsiree.
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