SAN DIEGO – The best day of match-racing in the America’s Cup regatta belonged to a mermaid ship and some old black magic.

Kevin Mahaney’s Young America, distinguished by a Roy Lichtenstein mermaid mural on its 75-foot hull, beat Dennis Conner’s struggling Stars & Stripes by 28 seconds Wednesday to take a commanding lead in the defender semifinals.

“Today was about the best boat race we’ve had all year,” Young America mainsheet trimmer Andreas Josenhans said. “It was just an awesome race. As many tactical moments as there were, we just got down into an absolute brawl.”

On the challenger course, Team New Zealand’s older yacht, Black Magic 1, displayed a superior speed advantage in overtaking John Bertrand’s year-old oneAustralia with about 4 1/2 miles to go and won by 39 seconds.

While the featured challenger race wasn’t decided until late, Mahaney and Conner sparred early and often.

There were three lead changes in the first half of the race before Young America went ahead for good to claim its second straight win this series and fifth overall.

After an even start, Stars & Stripes covered Young America’s wind and led by 45 seconds at the first mark. Young America rolled over Stars & Stripes going downwind, but Conner regained the lead and rounded the second mark 15 seconds ahead.

Going back upwind on the third leg, Young America won a downspeed tacking duel, got to the layline first and rounded the buoy seven seconds ahead, or just more than one boat length. Young America sealed the victory with a 29-second gain on the fifth leg of the six-leg, 18 1/2-mile course.

Stars & Stripes is slumping badly, losing its second race in the semis and its fifth overall. Young America is in first place in the Citizen Cup standings with four points, while Stars & Stripes remains tied with Mighty Mary with one point apiece.

“Obviously it’s not the happiest times for us in starting off the semifinals with two losses,” Stars & Stripes navigator Jim Brady said. “We felt good about today’s race in that the boat seemed to be going well with Young America and we had a very, very good race.”

Black Magic 1, meanwhile, lost the start to oneAustralia and instigated some of the most intense tacking duels so far on the challenger course. OneAustralia got two nice lifts from wind shifts, and led by 1:04 at the first mark.

But the Kiwis made up time at each of the next three marks to trail by only 10 seconds turning onto the windward fifth leg.

The yachts raced hull-to-hull on a long starboard tack, with oneAustralia to windward but Black Magic 1 showing a speed advantage.

OneAustralia was forced to tack away, and when the boats crossed again, Black Magic 1 had a three-length lead about halfway up the leg and built the advantage to 44 seconds at the mark.

“They eventually squeezed us off, went into a shift and got clear,” oneAustralia tactician Glenn Bourke said. “That was the race right there.”

“We wouldn’t be in the position we are now if we didn’t have a boat that’s a little bit quick,” Team New Zealand tactician Brad Butterworth said. “I wouldn’t say it’s demonstrably quicker than the others. We had a tough race with oneAustralia.”

Both syndicates are sailing their older yachts. The newer oneAustralia sank 2 1/2 weeks ago, and Team New Zealand decided that Black Magic 1 was close enough in peformance to Black Magic 2, which sailed in the first four rounds.

“She’s a good old girl,” Butterworth said.

Team New Zealand is 25-1, with its only loss coming in round two after a victory over oneAustralia was overturned on protest.

In the other challenger race, NZL-39 handed Nippon its third straight loss, by 57 seconds, to take sole possession of second place in the Louis Vuitton Cup..

Team New Zealand leads with three points, followed by NZL-39 with two and oneAustralia, one. Nippon has no points.