In St Louis in front of a crowd of around 75,000 according to police estimates, the opening stage consisted of ten 7.5 mile laps and was held on a muggy day.
Tomas Vaikus (Astanta), Chris Anker Sorensen (Saxo Bank) and Moises Aldape (Team Type 1) broke clear from the start of this street circuit 75 miles stage.
Sorensen who had done much to help Andy Schlek in the Tour De France, was the early points and polka dot jersey leader after winning the early sprints.
With a $15k Buell 1125R motorbike going to the winner of this stage, the lead of the front three went out to 1.15. Team Cervelo which included Brit Jeremy Hunt, Team Colombia and Saxo Bank chased down the leaders.
On lap 7, Vaikus and Sorensen were caught by the pack but Moises Aldape, who was voted the most agressive rider of the day last a quarter of a lap before he was caught.
Matthew Crane led as the race went into the last lap. Brad White and Sean Milne took over the running and ran up a ten second lead.
That was chased down by Colombia. Michael Rogers helped to stretch out the main field and with Cavendish sat behind the wheel of American Champion George Hincapie. Riders including Thor Hushovd tried to get on Cavendish's wheel.
However, there was confusion and a crash in the final kilometre but it was Cavendish who beat Hushovd to the line for his 24th victory of the season.
Cavendish won the Yellow jersey, Green Jersey and Best Young Rider.
He told his team's press office: "The guys did a great job. "Cervelo tried to give us a hard time at the end but the guys kept control and did a perfect job.
"George [Hincapie] fell down a hole in the road in the final, so I hope he's ok," he added. "I jumped on Thor's [Hushhovd] wheel and came round him."
"I didn't feel too good today due to the jetlag, but it was a pretty straight forward race and the guys kept it under control from the beginning."