Patten (right) finished behind Ilchenko for the second successive year
Cassandra Patten and Keri-Anne Payne qualified for the open water events at the Olympics after their displays at the 10km world championships.
The British duo claimed silver and eighth respectively in Seville to book their places at the Beijing Games.
Russian Larisa Ilchenko, 19, defended her 2007 title in a time of two hours, two minutes and 2.7 seconds.
Patten, 21, finished 3.1 secs behind to take silver for the second successive year with Spain's Yurema Requena third.
South African amputee Natalie Du Toit also qualified for Beijing after she finished fourth.
The 10km swim - often described as "wrestling in water" because of its aggressive tactics - will be making its debut at the Games in August.
"I am overjoyed with that result," said Patten, who narrowly missed out on gold in last year's event in Melbourne, where she had been stung by a jellyfish during the race.
I am now going to get back to the pool and work on my medley training and my gym work
And she was overjoyed to secure her second event at the Beijing Games, having already sealed her place in the 800m freestyle in the pool.
"I was really hurting in the final lap but I just thought that if I kept on I would become the first person to qualify for both the pool and open water and this is what I wanted so I kept myself going and got the second place.
"Last night I was really nervous, I didn't doubt myself but I knew that it would be a really tough race. I knew that one wrong move and I would get pushed back because of the quality of the field.
"Girls' races do tend to be closer than the men's races. I just wanted to be outside of the pack because it is a better when you have some space."
Payne, 20, slipped down the field as she got stuck in the pack but moved up the field to guarantee her place in the British Open Water Olympic squad with a top-10 finish.
"I planned to stick to the front of the pack," said the Stockport Metro swimmer, who has also qualified for the 200m and 400m individual medley events at the Olympics.
"I started well and I was in the top three for a long part of the race but there was a lot of fighting and I slipped back a bit, but I do think I should have stayed closer to the leaders.
"It is great to have qualified and I am pleased with the result. Pool swimming is my main focus and so to qualify in open water is a definite bonus. It is great to be one of the first people to qualify in both the pool and in open water.
"I am now going to get back to the pool and work on my medley training and my gym work to improve my fitness for future races."
Du Toit, who lost her left leg when she was hit by a car while riding her scooter in 2001, finished just 5.1 seconds behind Ilchenko.
The 24-year-old became the first amputee to race in the finals of a major able-bodied swimming competition when she made the final of the 800m freestyle at the 2002 Commonwealth Games.
She missed out on qualification for the 2004 Athens Olympics, but did compete in the Paralympics the same year where she won five gold medals and a silver.
Australia's two-time Olympic 1500m champion Grant Hackett is among the leading contender's in the men's 10km open water race at the Guadalquivir river course on Sunday.