Savinova, the Olympic and defending world champion, was roared round the two laps of the Luzhniki stadium but Sum timed her late burst through the middle to perfection, knocking almost two seconds of her personal best with a time of one minute 57.38 seconds.
The Russian was second in 1:57.80 and American Brenda Martinez snatched third (1:57.91) from her compatriot and long-time race leader Alysia Johnson Montano who threw herself over the line before dissolving into tears.
"When I saw Asbel win, I told myself let's also try," the 24-year-old Sum told reporters.
"In the last 100 metres, I saw I was really moving while the others seemed to be stuck.
"When I took the start I didn't know I was going to run like this or that I could be a winner. I was just hoping for a top five or maybe bronze," added Sum, who is more at home over 1,500.
"I think I will stick to the 800 now."
For once when 'shhhh' was played over the stadium's PA it was needed as a full house prepared to cheer Savinova to more glory.
Montano, fastest in the semi-finals, pulled away from the start and led by 10 metres at the bell but was clearly tying up as the athletes rounded the home bend and had nothing left to give in the dash for the line.
It was left to Savinova and Sum to battle for the gold and with the finish closing in, the Kenyan trials winner powered past her rival and threw her arms into the air as she crossed the line.
Savinova conceded she had under-estimated the little-known Kenyan.
"I didn't believe that Eunice Sum would be able to improve her PB by two seconds, it's huge in our event," the 27-year-old said.
"I just didn't believe she would endure that finishing sprint."
(Editing by Ed Osmond)
