LOS ANGELES—Jason Day is 29 years old, 12 years younger than his friend and mentor, Tiger Woods. But as Day watches Woods suffer through yet another chapter of his back-injury saga, his fear isn’t merely that he will end up like Woods at that age. His fear is that he will end up like Woods even sooner.

“Hopefully I can stay healthy and get to 40,” said Day, the world’s top-ranked golfer.

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