The Swiss boat surged ahead of the Kiwis following a tacking duel up the third leg before holding off the challengers on the final downwind lap for a 28-second win and a 4-2 series lead.

Only three times in the 156-year history of the cup has a series been this close. Emirates Team New Zealand needs to win the final three races to take the Auld Mug for the third time.

Alinghi can clinch the best-of-nine series with a victory in tomorrow's seventh race.

After trailing near the start, the Swiss slowly made ground on the Kiwis during the second leg, cutting their lead to 11 seconds around the marker as the boats set sail up the third lap.

Team New Zealand's advantage line on the flat water held despite the boats sailing far apart in a split.

But not for long.

Alinghi skipper Brad Butterworth — a three-time winner of the Auld Mug — came back at Kiwi helmsman Dean Barker as the SUI-100 boat surged ahead on the right of the NZL-92 yacht, rounding the top mark with a 16-second lead.

The Kiwis used a series of gybes to pull to within a boat length, but Alinghi stayed ahead by taking a more direct route over the final quarter. It was its third straight win after trailing the Kiwis 2-1.

The wind was steady but softer than Friday, when gusty winds put a strain on sails and cost the Kiwis, who lost two kites — and the lead — in a dramatic 19-second loss to the Swiss.

Alinghi helmsman Ed Baird held Barker from taking the right away as both boats got off the line even after an aggressive start.

A match race ensued up the first leg, just as it had all series.

Alinghi covered the Kiwis before tacking out to the right, Butterworth wanting to sail his own race.

But Barker followed, pulling ahead behind a stronger left-wind shift, before eventually nipping across the front of the SUI-100 yacht for a 14-second lead around the first marker.