Henrik Stenson partnered with an invitation-only golf society called The Underground and is playing their new ball at the Senior PGA Championship. (Photo by Mike Ehrmann/Getty Images)

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Henrik Stenson is making two big changes this week, making his senior circuit debut at the Senior PGA Championship and putting a new golf ball in play competitively after becoming the newest partner of an invitation-only golf society called The Underground.

Stenson turned 50 earlier this month, around the time The Underground began the first distribution of its premium golf balls to an exclusive membership that includes celebrities, C-suite executives, pro athletes from other sports, and other avid golfers eager to be a part of one of the game’s newest private clubs. Scarcity is part of the allure for The Underground, as its golf ball isn’t marketed or sold in stores.

The addition of Stenson is a significant milestone for The Underground, which is taking a niche approach in the golf equipment world by trying to build a community and product focused on performance rather than volume. Stenson, who spent the past four years on the LIV Golf tour, was a member of three Ryder Cup-winning teams for Europe and in 2013 made history by becoming the first golfer to win both the FedEx Cup and the Race to Dubai in the same season.

“We built The Underground for players who care about performance at the highest level, but also want to be part of something that sits outside the mainstream,” said Garry Singer, who co-founded The Underground with actor/producer Mark Wahlberg and retail magnate/philanthropist Doug Meijer. “To have a major champion like Henrik put the ball in play and trust it when it matters most says everything about what we’ve built.”

Stenson, the 2016 Open champion, is one of 21 major winners in the field at the Senior PGA Championship, which is being contested from April 16-19 at The Concession Golf Club in Sarasota-Bradenton, Florida.

“I can feel the difference in quality and consistency," said Stenson, who developed a reputation over the years as a precise ball-striker. “I’ve put in the time testing it, I trust it, and I’m really looking forward to getting it out there in competition.”

The Underground occupies a decidedly unusual space within the golf world. Rather than retail rollouts or challenging legacy golf ball brands, the group – which got its start when its well-connected founders decided they wanted to create the best ball possible just for themselves – has an application process to join.

If selected to be a part of The Underground, members get limited-edition apparel, accessories and gear (including some branded “drip” from Wahlberg’s Municipal brand), but the hook is a golf ball that its founders say is held to a level of quality control that exceeds the standards of the game’s largest manufacturers. Members receive an annual allocation of 12 dozen golf balls, the same ones Stenson will be putting to the competitive test this week at the Senior PGA Championship.