Walker defied the wet and windy conditions to reel in Morocco's Abdelati El Guesse and South Africa's Rynardt van Rensburg on the closing straight and take a surprise gold medal at the Summer Universiade games.
Despite languishing back in fourth place after the first lap, Walker triumphed in a time of 1min 49.05sec to become the first American to win the 800m Universiate title since Evans White in Mexico City in 1979.
On the penultimate full day of action at Gwangju 2015, hosts South Korea pulled further ahead in the overall medal table thanks - most notably - to a clean-sweep of all five badminton finals, victory in the men's single tennis by 19-year-old Chung Hyeon and a rare defeat inflicted on Russia by Yeon Jae Son in the men's individual rhythmic gymnastics.
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South Korea now have 44 gold medals - 12 more than both Russia and China - and are all but guaranteed in topping the table.
Elsewhere in a packed athletics programme, Azerbaijan's Hayle Ibrahimov defended his 5,000m title while becoming the only man to break the 14-minute barrier, striding to gold in a time of 13:44.28.
Docus Agok comfortably won the women's 1500m while Russia's dominance continued with long jumper Pavel Shalin leaping 8.29 metres en route to another gold medal.
In the relay competitions, Kazakhstan won the women's 4x100m to give Viktoiya Zyabina her third gold medal of the games, while Japan's men claimed the 4x100m crown.
The men's 4x400m event was won by the Dominican Republic - led by 400m gold medallist Leguelin Santos - while Poland caused an upset by stifling out both the US and Russia in the women's 4x400m final.
Outside the athletics stadium, there was a second successive volleyball gold medal for Russia over Ukraine as the men's team picked up where the women's left off with a 3-1 victory. After edging the first two matches 27-25 and 25-23, Russia lost the third match 25-21 before wrapping things up with an emphatic 25-14 win in the fourth.
The first of the football finals saw France beat Russia 2-0 after a second-half strike from Valerie Gauvin was followed by an own goal by Liubov Kipiatkova. Italy play hosts South Korea in the men's final at the Naju Public Stadium on Monday.
In table tennis Chinese Taipei were beaten 4-2 in the women's doubles by China before the men's doubles team turned things round with a 4-1 win over Japan in their final.
And finally, there were taekwondo gold medals for Belgium's Jaouad Achab, Korea's Kim Minjeong, Iran's Amidi Omid and China's Shuyin Zheng.
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