Two-time defending champion Tom Slingsby and his Team Australia have enjoyed finishes of first, third and first at SailGP's San Francisco regatta as his Flying Roo prepares for a third-straight grand final.

Slingsby is the only skipper to have already qualified for Sunday's $US1 million ($A1.5m) winner-takes-all showpiece race.

"It's the day we needed and it's given us that confidence that we have missed at previous events," Slingsby said after a dominant session of racing.

"It's $1 million for first and $0 for second, so there is no second tomorrow."

A near-collision with Team Canada on Saturday briefly left Slingsby fearing the worst but the skipper's quick reactions prevented a potential regatta-ending shunt.

"I don't know if they didn't see us or they didn't react in time but that was a really close call," a relieved Slingsby said.

Peter Burling and Team New Zealand are in good shape in second place in the overall season standings.

British sailing star Sir Ben Ainslie strengthened his chances of reaching the grand final with finishes of second, first and second on a San Francisco course charting the city coastline between the Golden Gate Bridge and Alcatraz Island.

Ainslie's Emirates Great Britain team came into the regatta in fourth place in the season standings, one point behind France's Quentin Delapierre, who was constantly outmanoeuvred by the British skipper and had finishes of sixth, eighth and sixth in the nine-boat fleet of foiling 50-foot catamarans.

The top-three crews after Sunday's final two fleet races will advance to the grand final of tech tycoon Larry Ellison's global league.

Ainslie is second at the regatta, behind Slingsby on 28 points, with Burling third with 19 points after finishes of fourth, seventh and third.

Burling, the two-time reigning America's Cup champion helmsman, needs to finish no lower than fifth in San Francisco to claim a spot in the grand final.

"We were not on a good level today," Delapierre said.

Ainslie, a master at comebacks during his brilliant career, relentlessly put pressure on the French crew by blocking it during pre-starts and forcing their catamaran to fall off its foils.

"The team is working hard on executing the manoeuvres," Ainslie said.

"We had a wobble in the first race, a cleaner one in the second. So we've just got to keep trying to do that and put pressure on the French."

Ainslie is the most-decorated sailor in Olympic history, having won four gold medals and one silver.

He was crewmates with Slingsby aboard the Ellison-owned Oracle Team USA, which successfully defended the America's Cup in 2013 with a comeback against Emirates Team New Zealand on San Francisco Bay.

Ainslie's SailGP strategist Hannah Mills is the most successful female Olympic sailor of all time with two golds and a silver.