Complex sentence: It was this US Open, that Phil Mickelson declared that this would be his last tournament, before he took significant time off to tend to his ailing wife, Amy, who had been diagnosed with breast cancer.
Split into simpler sentences: It was this US Open, that Phil Mickleson declared that this would be his last tournament, before he took significant time off to tend to his ailing wife. This was due to the announcment that Mickelson's wife, Amy, had been diagnosed with breast cancer.