Tomorrow, the greatest threat is yet another American, Jason Lezak, who is ranked eighth in the world.
Foster, spurred on by last month's European silver medal in Valencia, said: "I wasn't rested at the European Championships and still did a good time, so it gives me extra confidence I can get the world record back in Sheffield. That's what really matters now. All my training since the Olympic final in Sydney has been to get back this record."
The Fina World Cup starts today at the Pond's Forge pool and Foster, with gold in the 50m butterfly at the Europeans last month and the world short-course title to his name, is also favourite this week to win the 50m butterfly.
Joint British record-holders in the 50m breaststroke, James Gibson from Loughborough University, and Darren Mew from Bath University, are up against Germany's Mark Warnecke, who holds the world short-course record at 26.70secs.
In the 100m breaststroke, Warnecke has recorded a time of 59.60secs, but Darren Mew's best is slightly quicker at 59.45secs.
Mew admitted: "I'll be racing hard in Sheffield. Ever since I was a teenager in the age group competitions I've always known that Warnecke is the man to beat in the 25m pool, particularly over 50m.
"I was thrilled with my time in the European Championships and yes there is rivalry between James Gibson and myself. We both want to hold the British record in our own right - who wouldn't?"
In the women's events, Olympic champions Brooke Bennett and Megan Quann attract the greatest attention.
Diminutive American Quann, who turns just 17 this week, won gold in Sydney in the 100m breaststroke. She's up against team-mate Katie McClelland and Australia's Brooke Hanson.
Bennett is Olympic champion in the 400m and 800m freestyle. At 20 years old, many believe she is the only swimmer capable of breaking fellow American Janet Evans's long-standing long-course (50m pool) world records.
Stacey Houldsworth from Lancashire is ranked second behind Bennett for both the 400m and 800m this week and is swimming well.
