He'll be sailing in home waters in front of thousands of fans in this quaint seaside resort that was favoured by King George III. And he's the odds-on favourite, still as driven and focused as ever at 35.
Opening the Olympic regatta with the marquee act is appropriate as Ainslie, the son of a sailor, tries for his fourth straight gold medal and fifth straight medal overall.
The expectations will be enormous.
That's OK with Big Ben.
"I think I perform well under pressure, because, in an odd way, it gives me a wake-up call," said Ainslie, who lit the cauldron at the venue on Saturday afternoon.
Ainslie said he's driven simply by the chance to win another gold, not necessarily by surpassing Paul Elvstrom of Denmark as the greatest Olympic sailor ever. Elvstrom won four straight gold medals from 1948-60.
"A home Olympics makes it very special," Ainslie said. "I was in Trafalgar Square when they announced that London had won the bid for 2012. The atmosphere was electric and that's when I decided I wanted to continue my Olympic career and be a part of it."
Ainslie, 35, is the most famous member of Britain's powerful and well-funded sailing team — a nautical equivalent of the New York Yankees — which has topped the medals tables at the last three games. He's been called Britain's greatest sailor since Admiral Lord Nelson, whose statue high above London's Trafalgar Square commemorates victory over the French and Spanish fleets in 1805.
Nelson paid for his glory with his life at the Battle of Trafalgar, felled by a musket shot from the French ship Redoubtable.
Ainslie faces no such danger, although the 23 other Finn sailors will be more than eager to gain their own Olympic glory and the chance to take down one of their sport's greatest.
Ainslie takes nothing for granted and it will be a big task to beat him.
He hasn't lost at an Olympics since 1996. Making his games debut as a 19-year-old, he had to settle for silver in a bitter loss to Brazil's Robert Scheidt in the Laser class. Since then, Ainslie has been focused, driven and even merciless. After expertly extracting revenge against Scheidt to take gold at Sydney, Ainslie moved up to the Finn, a heavyweight dinghy that takes more brute strength to sail than any other Olympic class, and won the gold at Athens and Beijing.
He's one of Britain's poster boys for these games. Despite a shocking outburst at a TV cameraman that got him disqualified from the world championships in Australia in December, Ainslie was accorded the honour of beginning the Olympic torch relay.
Among those with a chance to bring down Big Ben is Zach Railey of Clearwater, Fla., who took silver in Beijing in his Olympic debut.
"It's going to take me sailing extremely well," Railey said. "But I'm confident in my abilities."
Besides facing off against Ainslie, Railey gets to sail in the Olympics with sister Paige, who will represent the United States in the Laser Radial.
The venerable Star class, which also begins sailing Sunday, will provide a heavyweight showdown. Britain's Iain Percy and Scheidt, who finished 1-2 in Beijing, are back for another go-round. Scheidt is the reigning world champion and has been sailing well.
Scheidt has won two golds and two silvers in his Olympic career, the first three in the Laser class before moving up to the Star. Percy also has two golds, including in the Finn in 2000.
Scheidt expects a competitive regatta.
"I enjoy it because the level is so high," he said. "To win a race or to do well you've got to really hit the shifts and start well and sail well upwind and downwind. It's very exciting because it's as high as you can get, and it should be like this."
The Finn and Star classes are scheduled to have two races Sunday. The medals races in both classes are on Aug. 5.
Women's match racing begins Sunday as well. The favourite is Anna Tunnicliffe, who was born in England and moved with her family to the United States when she was 12. She won the gold in the Laser Radial in 2008. Now living in Plantation, Fla., she skippers a boat that includes crew Molly Vandemoer of Stanford, Calif., and Debbie Capozzi of Bayport, N.Y.
The regatta will be held on Weymouth Bay and Portland Harbor, which served as the embarkation point for the majority of the American soldiers who landed in Normandy on D-Day.