BROOKLINE: A hotly anticipated US Open starts Thursday that will test the world’s top golfers over a formidable layout at a historic setting with a splash of controversy as well. Defending champion Jon Rahm, four-time major winner Rory McIlroy, top-ranked Masters champion Scottie Scheffler and two-time major winners Justin Thomas and Collin Morikawa lead fan and oddsmakers favorites at The Country Club. In addition to creating dense rough and lightning-fast greens, the US Golf Association ensured drama by allowing players from the upstart Saudi-backed LIV Golf Series to compete rather ban them as the US PGA Tour did. As a result, US stars Dustin Johnson, Phil Mickelson, Patrick Reed and Bryson DeChambeau are among the LIV Golf rebels teeing off in a unique sub-plot to the quest for a major title —the first LIV-PGA showdown. Mickelson, who turns 52 on Thursday, would complete a career Grand Slam with a victory after a record six runner-up efforts, and break his own mark as the oldest major winner. “I’m excited to be back in this incredible championship that has eluded me,” Mickelson said. Boston fans famously heckled Europe in a 1999 US Ryder Cup fightback victory and could offer more barbs. Tiger Woods, making a comeback after severe leg injuries suffered in a car crash 16 months ago, skipped the US Open to better prepare for next month’s British Open at St. Andrews. A field of 156 will compete at the same club where 20-year-old hometown hero Francis Ouimet became the first amateur to win the US Open back in 1913. Third-ranked McIlroy won his 21st US PGA title on Sunday at the Canadian Open ” McIlroy, 33, is the most recent player to win a tour event and a major title in back-to-back weeks, taking a WGC title at Akron before his 2014 PGA Championship victory at Valhalla, his most recent major triumph. Scrambling for momentum: Second-ranked Rahm will try to repeat as Brooks Koepka did in 2017-2018. The 27-year-old Spaniard won his first major title last year at Torrey Pines. “A lot of the pressure I used to put on myself is not really there,” Rahm said. “I can enjoy it a little bit more and know you don’t need to do anything special to get it done.” Rahm won his first title since last year’s US Open last month at the US PGA Mexico Open and practiced Tuesday with Mickelson. Scheffler, assured of topping next week’s rankings, will try to match Woods — the only reigning world number one to win a US Open. Thomas, who won May’s PGA Championship, likes the difficult test on offer.