Oudin, a 17-year-old southern belle from Georgia, USA, was too hot for the sixth-seeded former world No.1 to handle, winning 6-7, 7-5, 6-2.
Later, Serb Jankovic blamed her monthly period, not the 100-degree heat, for the upset. She said: "What can I do? It was woman's trouble.
"It's not easy being a woman sometimes. I felt really dizzy, tired and exhausted and thought I was going to end up in hospital.
"I started to shake and I was losing consciousness."
Jankovic all but collapsed at the end of the first set, which she edged in a tense tie-break.
The Serb called for the Wimbledon physio and stretched out on the Court Three grass while the medic took her blood pressure and placed an ice pack around her shoulders. Jankovic's problems did not take any joy away from her American conqueror.
Oudin beamed: "I'm thrilled and I think I handled it very well. I was just thinking she was any other player and this was any other match and I was at any other tournament - not like on the biggest stage at Wimbledon playing my first top-10 player."
The storming win will catapult the pony-tailed blonde into the world's top 100 - not bad for a girl who began the event ranked at 124.
She is also guaranteed £53,250 in winnings - around $90,000 - which is quite a windfall for the teen whose career earnings before this week were $135,000.
And keep an eye on her. She is in scorching form with 20 wins in her last 22 matches and fancies her chances of more upsets to come.
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