NAPLES, Florida — Jason Day and Lydia Ko stormed to the lead on Saturday on account of their back-nine 31 in the foursomes format of the Grant Thorton Invitational, a mixed-team event with changing formats every round.

They finished the second round with a six-under-par 66 for a two-shot lead that was highlighted by five birdies on the back nine at Tiburon.

Day and Ko are now at 20-under 144, two ahead of Lucas Glover and Leona Maguire (69) and Tony Finau and Nelly Korda (70).

"I think the biggest thing in foursomes is to keep the ball in play," Day said.

"My personal goal was to just get it down the fairway, keep it in play. …Lydia, she's been a solid rock for the team."

Rickie Fowler and Lexi Thompson assembled a stunning back nine, too, underscored by Thompson's hole-in-one on the par-3 16th hole.

But they bogeyed two of the par 5s against four birdies and Thompson's ace. They had a 31 at the turn en route to a 68.

The popular tandem is four shots behind in a tie with the Canadians Corey Conners and Brooke Henderson (69); and Nick Taylor and Angel Yin (70).

Sahith Theegala and Rose Zhang were tied with the Finau-Korda team but limped home with a 74, eight shots away.

Finau and Korda were in the lead most of the day. But Finau bungled a chip on the par-5 14th. Korda failed to find the green before missing a six-foot par putt.

Thompson was staring at 158-yard hole clutching a 7-iron — two clubs high to score the ace.

"An amazing experience," Thompson said.

The mixed teams — one PGA Tour, one from the LPGA — is the first such event since the JC Penney Classic in 1999.

The final round is modified fourballs, new to the event. Both players hit tee shots, and they switch golf balls for the second shot and each play the ball until conclusion.