By Alan Baldwin

RIO DE JANEIRO (Reuters) - Michael Phelps is fired up and ready to go for what promises to be an explosive first weekend of the Rio Olympic swimming program.

The American could feature in Sunday's 4x100 freestyle relay final and should collect the 23rd Olympic medal of his record-breaking career if he does.

The U.S. men have not failed to finish in the top three since the event was introduced in 1964 but champions France and resurgent Australia will be tough opponents.

The women's 4x100 is Saturday's late night highlight, with U.S. golden girl Katie Ledecky tipped to swim in the preliminary heats to qualify for a medal in an event world champions Australia are favorites to win.

"The first two nights of the relays, with the women’s on the first night and the men’s on the second, there will probably be a lot of fireworks in the swimming pool," said Phelps.

"I’m looking forward to either watching or being in that race. Those races are ... super fast and there are always some crazy splits that take place."

Water and pyrotechnics usually result in a damp squib but Phelps is right to be excited. The sprint relays offer memorable nights of high drama.

Now 31 and in his fifth Games, the most medaled Olympian of all time (18 golds) has played his part in many such moments but may have to dig deeper this time.

"I think this sport has changed to where there’s not really one or two power houses," said Phelps. "You could probably pick three or four teams that are going to have a chance to win that (men's) relay."

"The Australians have made a significant charge over the last couple of years ... there are a lot of young guys who have really stepped up that I think brings more excitement to the sport."

Russia took the bronze in 2012 but two of that quartet -- Vladimir Morozov and Nikita Lobintsev -- will be absent due to the doping scandal that has overshadowed the run-up to Rio.

The shadow of the past also hangs over Saturday's men's 400 freestyle, won in 2012 by China's Sun Yang ahead of South Korea's Park Tae-hwan.

Park, the first Korean to win an Olympic swimming medal when he claimed gold in Beijing in 2008, completed an 18-month ban imposed by world governing body FINA in March after testing positive for testosterone.

Sun served a three-month ban in 2014 after testing positive for a banned stimulant, spent a week in jail for crashing a car driven without a license, and was involved in an altercation at last year's world Championships.

(Reporting by Alan Baldwin; Editing by Andrew Both)