Michael Phelps's bid to tie the record of seven gold medals at a single Olympics on Day 8 will likely face the greatest opposition, if any, from two swimmers born in America, one of whom is representing Serbia.

Phelps can tie the record established by Mark Spitz at the 1972 Munich Games with a win in the 100-metre butterfly at the Beijing Aquatic Center. The final is set for 10:10 p.m. ET on Friday.

Michael Phelps, right, has usually beaten rival Ian Crocker, left, though not at the 2005 world championships in Montreal. (Al Bello/Getty Images)

Phelps made it six gold medals — and six world records — blowing away Ryan Lochte and the rest of competition in the 200-metre individual medley on Day 7.

Not that Phelps needed the help, but Lochte had just won gold in the 200 backstroke in world-record time about a half hour earlier.

Most observers are eyeing countryman and current world 100m butterfly record holder Ian Crocker as the man who can derail the quest for history, but not Gary Hall Jr.

Hall, who has won 10 Olympic medals and five gold in his career but didn't qualify this year, is picking Milorad Cavic of Serbia.

Cavic set an Olympic record in the preliminary heat with a time of 50.76 seconds. He also had the top time in the semis, at 50.92.

The Serb's result helps heighten interest for the final even more after Phelps and Crocker had collectively held the top 17 swims ever in the event heading into Beijing.

"An upset would be the upset of all upsets, it’s true, but I think Mike [Milorad] can beat Michael," Hall said Friday in a column for the Los Angeles Times.

Cavic, 23, was born in Anaheim and attended the University of California at Berkeley.Olympic champ Gary Hall Jr. thinks Milorad Cavic, pictured, can pull off the upset. (Jonathan Gerrey/Getty Images)

The six-foot-five Cavic, who has battled back problems in the past, said in a posting on his website Friday: "So here I am, in the eve of battle, feeling physically better than ever with a chance to show the world and myself what I’ve worked so hard for. I’m feeling good … and I’m excited, so here we go."

Baltimore native Phelps may not be feeling "better than ever" but he appears in as good shape as a man can be after 15 elite races in seven days.

Crocker looking to turn tide

Within an hour of winning medal No. 6, Phelps came back to win his semifinal butterfly heat in a time of 50.97 seconds.

"I have to conserve as much physical and emotional energy as I can now that I'm down to the last two races," Phelps told reporters afterward.

The potential record-breaking race for Phelps is Sunday when he is due to take part in the men's 4x100 medley for the United States.

Crocker beat Phelps at the 2003 and 2005 world championships, setting a world record in the latter meet in Montreal in the process (50.40).

Recent history, however, hasn't suggested Crocker will pull off the Olympic upset, despite this being his only event in Beijing.

The two were involved in a similar scenario at the U.S. Olympic trials in Omaha, Neb. In the last of his eight events at the trials, Phelps was the one fresher at the end, overtaking a brief Crocker lead to win in a time of 50.89.

The turning point in the rivalry between them seems to have been the 2007 championships in Melbourne. Phelps edged Crocker by 0.05 seconds to win the butterfly, and heading into Beijing had posted the three fastest times of 2008.

Crocker said he is not getting caught up in history or past results.

"You can start by not worrying about what everybody else thinks," Crocker said. "Nobody knows what I've really gone through in the last eight years and what has gotten me to this point, besides myself and a few people that I know well. So it's my own personal deal at this point."

Crocker was second to Phelps in the semifinal heat despite being the more rested, but it may well have been a tactical move. The Portland, Me., native shared the third-best qualifying time, with Australia's Andrew Lauterstein.

Spitz has been complimentary of Phelps although he has also said he would have taken eight gold in Munich had the 50m freestyle been in existence. Of the many swimmers Spitz defeated at the 1972 Games, one was Gary Hall Sr.

With two days of swimming left, the powerful U.S. team has piled up 25 medals, including 10 golds.

With files from the Associated Press