The Sony Open Tennis Tournament, considered the sport’s “fifth Grand Slam” event, wants its county-owned Crandon Park home to get a $50-million makeover with new permanent grandstands, shaded pavilions, a grassy lawn and multi-story stadium additions.

The reason? Tournament organizers say the tennis center can no longer compete with other cities, raising the possibility — however remote — that, when the event’s lease runs out in nine years, the tournament could leave Miami.

That’s the campaign pitch organizers will make to Miami-Dade voters, if county commissioners agree on Thursday to put the question on the Nov. 6 ballot.

Two-thirds of voters would have to approve the new and expanded park structures, which would be financed by tournament revenues. In return, the tournament wants its lease extended to a total of 30 years with two optional, 10-year extensions.

But when it comes to Crandon Park, change has rarely come easy.

If approved by voters, the project would require support from a special committee that signs off on any major changes to Crandon Park, and also would need to meet terms set by the pioneering Matheson family that donated the land to the county and later sued Miami-Dade when it planned to build the tennis stadium.

Tournament organizers say if Miami doesn’t keep up with facilities in places like Madrid and Shanghai, the Sony Open (formerly known as the Sony Ericsson Open), which draws more than 326,000 visitors a year, could lose its luster.

“Events are catching up,” said Adam Barrett, senior vice president of IMG, which runs the tournament. “We want to be the leader. We want to be world class.”

The upgrades would be financed by private tournament funds and tennis center and tournament revenues, including parking fees and ticket surcharges, without a reduction to the tournament’s annual contributions to county coffers. Were Miami-Dade to issue revenue bonds to finance the project, they would be backed by tournament revenues, not the county’s general fund, Barrett said.

A county charter revision adopted in 1993 known as “Save Our Parks” requires voters to approve large, permanent structures or private, commercial uses at many county parks. The restriction is even stricter for parks like Crandon, where a two-thirds majority of the vote is required.

The measure was partly a result of years of tumult surrounding Crandon, a nearly 1,000-acre park on the northern tip of Key Biscayne. The Matheson family deeded the land in 1940 in exchange for the county building the Rickenbacker Causeway — with the condition that the land be used only as a public park.

The family sued in 1991 after the county planned to build a tennis stadium for the tournament, then known as the Lipton International Players Championship. Four years later, the two sides reached a settlement that stipulated, among other things, that the county could proceed as long as it set up a committee to approve future changes, giving the family a say. Bruce Matheson remains on the four-member committee.

Matheson said late Wednesday he wants more details about the tournament’s proposal, which he learned of only days ago.

It appears that the tournament “would like to add three more permanent stadiums to the Crandon Park Tennis Center,” he said, referring to the stadium and grandstand expansions.