Ye Shiwen, 16, is chasing a second gold in the 200 metres individual medley final after winning the 400 medley during the weekend, more than a second inside the world record.
"We want to be very careful about calling it doping," the American John Leonard, executive director of the World Swimming Coaches Association, told Britain's Guardian newspaper.
"The one thing I will say is that history in our sport will tell you that every time we see something - and I will put quotation marks around this - 'unbelievable', history shows us that it turns out later on there was doping involved."
China briefly dominated women's swimming in the 1990s but its reign ended as fast as it began, following a series of doping scandals.
Ye swam the 400 medley five seconds inside her personal best, covering the last 50 metres faster than American Ryan Lochte, who won the equivalent men's event in the second best time in history.
She issued a quick and firm denial yesterday, telling the China News Service: "My results come from hard work and training and I would never use any banned drugs."
Australia's Ian Thorpe, winner of five Olympic swimming golds, warned against rushing to judgment.
"Young swimmers can take off chunks of time that other swimmers can't," he said.
"RUINING SPORT"
International Olympic Committee medical chief Arne Ljungqvist said it would ruin the "charm of sport" to raise doping suspicions every time an athlete's performance improved dramatically.
Others noted that Phelps had broken his first world record at 15.
"Michael Phelps is a phenomenal swimmer," British multiple short-course world champion Mark Foster said. "Is she the Chinese Michael Phelps? Why not?"
American Phelps has gone on to win 17 Olympic medals, 14 of them gold. If he wins two more in tomorrow's 200m butterfly and 4x200m relay, he will overtake Soviet gymnast Larisa Latynina's record haul of 18.
He also has the chance in the butterfly, his favourite event, of becoming the first man to win the same swimming event in three successive Olympics.
"I made my first Olympic team in this. The shorter races are a lot better for me now that I'm older," Phelps said.
Today provided another story of youthful success in the pool as Lithuania's swimmer Ruta Meilutyte, just 15, won the women's 100 breaststroke, and an upset as Frenchman Yannick Agnel beat Lochte in the men's 200 freestyle.
A total of three swimming golds put France third in the medal table at the end of Monday's third day of competition, behind the United States, on five golds, and China, on nine.
For the host nation, golds are proving elusive but a bronze in the men's team gymnastics felt almost as good as it ended a 100-year wait for any kind of a medal in the event.
The focus of home attention tomorrow is Wimbledon as Andy Murray competes in the second round of the tennis competition, once again carrying the hopes of British fans yearning for a title after his final defeat to Roger Federer in the grand slam tournament there earlier this month.
Women's soccer throws up a tasty tie between North Korea and the United States, at Manchester United's Old Trafford ground, that may prove as much of a spectacle for students of Cold War rivalry as for die-hard sports fans.
