BROOKLINE, Mass. – Jordan Spieth will have to gut out a U.S. Open victory – literally.

As reported by Golf Channel's Steve Burkowski, Spieth is battling a stomach bug entering Thursday's first round at The Country Club, where Spieth began his championship at 7:29 a.m. ET.

Spieth started feeling ill on Tuesday morning and got through three balls on the range before stopping. Spieth's practice was limited each of the last two days, as he mostly putted and chipped around the course while feeling "about 20%," according to Burkowski.

Spieth's camp said that Spieth is feeling better on Thursday morning and hopes to be about 70-75% for Round 1.

Earlier this year, Spieth missed the cut at the Farmers Insurance Open while battling another digestive issue. The next week he went on to finish runner-up at Pebble Beach.

"If he's upright," caddie Michael Greller said, "he's playing,"