Holders Alinghi bounced back in the America's Cup on Wednesday and levelled the best-of-nine series 2-2, but then faced a protest from the Kiwis about their boat's rig.

Team New Zealand lodged the protest against Alinghi over the way its mainsail is attached when it is hoisted up the mast.

Earlier, race officials had inspected Alinghi's SUI100 and Team New Zealand's NZL92 as the boats returned to port following a race which Alinghi had led from the start.

"The jury will hear the protest regarding the halyard lock at 0900 GMT on Thursday (Thursday 9pm NZT)," an America's Cup official said.

The race jury can impose a range of sanctions if a team is found to have violated a technical regulation, ranging from a fine to deducting a point, a resail or ultimately disqualification from the America's Cup.

Friday is a "lay" or rest day for the crews, with racing scheduled to resume at 1pm on Saturday.