That's all Phelps needed. He didn't have to be the fastest. Just first.
Phelps swam into history with a magnificent finish Saturday, tying Mark Spitz with his seventh gold medal by the narrowest of margins in the 100-meter butterfly.
He got his hands on the wall a hundredth of a second ahead of Cavic - a finish so close the Serbians filed a protest and swimming's governing body had to review the tape down to the 10-thousandth of a second.
"I had no idea," Phelps said. "I was starting to hurt a little bit with probably the last 10 meters. That was my last individual race, so I was just trying to finish as strong as I could."
Phelps' time was 50.58 seconds, the only time in these Olympics that he won an event without breaking the world record. Not to worry. The 23-year-old from Baltimore has now pulled even with the greatest of Olympic records, matching Spitz's performance in the 1972 Munich Games.
Call this one the Great Haul of China - and it's not done yet. Phelps will return on Sunday to swim in his final event of these games, taking the butterfly leg of the 400 medley relay. The Americans will be heavily favored to give him his eighth gold, leaving Spitz behind.
Phelps slapped his hands on the water and let out a scream after the astonishing finish. The crowd at the Water Cube gasped - it looked as though Cavic had won - then roared when the "1" popped up beside the American's name.
Cavic's time was 50.59.
The Serbian delegation filed a protest, but conceded that Phelps won after reviewing the tape provided by FINA, swimming's governing body. USA Swimming spokeswoman Jamie Olson said the tape was slowed to one frame every 10-thousandth of a second to make sure Phelps actually touched first.
It was impossible to tell on regular-speed replays.
"We filed the protest but it is already over," said Branislav Jevtic, Serbia's chief of mission for all sports. "They examined the video and I think the case is closed. The video says (Phelps) finished first.
"In my opinion, it's not right, but we must follow the rules. Everybody saw what happened."
FINA referee Ben Ekumbo of Kenya said there was no doubt who won after a review of the super-slow replay.
"It was very clear that the Serbian swimmer touched second after Michael Phelps," he said. "One was stroking and one was gliding."
Cavic wasn't sure he actually lost, but said he would accept FINA's ruling.
"I'm stoked with what happened," Cavic said. "I don't want to fight this. People will be bringing this up for years and saying you won that race. If we got to do this again, I would win it." Cavic watched the replay himself.
"It's kind of hard to see," he said. "I know I had a long finish and Michael Phelps had a short finish."
A notoriously slow starter - Phelps was seventh out of eight at the turn - he really turned it on with the return lap, his long arms gobbling up huge chunks of water as he closed the gap on Cavic and fellow American Ian Crocker, the world record-holder.
As they approached the finish, with Phelps' head in line with Cavic's shoulder, the Serb took his final big stroke and glided underwater toward the gold. Phelps, his timing a bit off but fully aware of where he was, did another mini-stroke, propelling his upper body out of the water, swooping his arms in a huge circular motion and slamming the wall with his hands on the follow-through.
"I actually thought when I did take that half-stroke, I thought I lost the race there, but I guess that was the difference in the race," Phelps said.
It was reminiscent of the 100 fly finish at Athens four years ago, where Crocker appeared to have the race won but Phelps got him at the wall by 0.04.
"My last two Olympics I've been able to nail my finishes, and it's been by four one-hundredths and one one-hundredths," he said. "I'm happy and kind of at a loss for words."
Phelps set world records in his first six events, some of them by ridiculously large margins. He merely settled for a personal best in the 100 fly, which will at least give Spitz's supporters some reason to gloat: all seven of his wins in Munich were with world records.
But, like Spitz, Phelps is 7-for-7 with a chance for one more. Or maybe that should be 6.99-for-7.
Andrew Lauterstein of Australia won the bronze medal in 51.12. Crocker was again denied the first individual gold of his career; he didn't even win a medal, finishing fourth by a hundredth of a second in 51.13.
"It was a tight one," Crocker said. "I saw my short differential between getting a medal or not, but then I realized Michael's was pretty close, too. I'm really glad that he came out on top.
"It was everything that an Olympic final should be. It doesn't matter who's in the heats, you just got to get out and race and it's anybody's game. It was one of the more intense races that I've been in, which makes it a great way to end the meet."
While the medley relay figures to be nothing more than a coronation, Phelps isn't ready to talk about No. 8.
"It's not over yet," he said. "I really think the Australian team looks great for the relay. It's going to be a race."
Lauterstein was just thrilled to be part of history.
"It was an amazing final," he said. "Every time you race Phelps, you'll have a great race and a great time. Just hearing his arms slap on the block gets your heart racing, he's amazing. I'm so happy to get the medal."
Overshadowed by Phelps, two more world records fell on the next-to-last day of swimming - the 22nd and 23rd of a lightning-quick week.
Rebecca Adlington of Britain won gold in the 800 freestyle, breaking Janet Evans' 19-year-old world record - the oldest in swimming.
Adlington touched in 8:14.10 to crush the mark of 8:16.22 set by the American in Tokyo on Aug. 20, 1989. Alessia Filippi of Italy took the silver and Lotte Friis of Denmark the bronze.
Adlington completed a sweep of the women's distance events in Beijing, having upset American Katie Hoff to win the 400 freestyle. There were no Americans in the field after Hoff and Kate Ziegler were shockingly eliminated in the preliminaries.
Kirsty Coventry of Zimbabwe finally won a gold in Beijing, defending her Olympic title in the 200 backstroke with a world record of 2:05.24. She lowered the mark of 2:06.09 set by Margaret Hoelzer at the U.S. trials last month.
Hoelzer not only lost her record but had to settle for silver. Reiko Nakamura of Japan earned the bronze.
No one was happier that Cesar Cielo, who won Brazil's first swimming gold with an upset in the 50 freestyle. He broke down crying on the medal stand and was mobbed by his teammates on deck.
He won in 21.30, lowering his own Olympic mark of 21.34 that he set in the semifinals. Amaury Leveaux of France took the silver in 21.45. Alain Bernard of France, the 100-meter champion, won bronze in 21.49.
World champion Ben Wildman-Tobriner of the United States was fifth and Aussie Eamon Sullivan, the world record-holder and silver medalist in the 100, could only manage sixth.
Also, 41-year-old Dara Torres of the U.S. cruised into the final of the 50 free with the fastest semifinal time, 24.27. Australian teenager Cate Campbell was second at 24.42.
The final is Sunday, the wrapup to a thrilling competition at the Water Cube.