Jordan Spieth made a near-flawless start in his debut appearance at the Travelers Championship as the former world number one earned a one-stroke lead, while Jason Day struggled.

Competing at TPC River Highlands for the first time in his career, two-time major champion Spieth carded a seven-under-par 63 during Thursday's opening round.

After finishing tied for 35th at last week's U.S. Open, Spieth's short game was on song as he posted eight birdies and only dropped one shot at the ninth hole.

It was Spieth's lowest opening round of his career, which saw the American end the day ahead of countrymen Johnson Wagner and Brett Stegmaier.

Graham DeLaet and Troy Merritt both carded 65s to be two shots off the pace at five under in Connecticut.

Xander Schauffele continued to make waves on Thursday via his four-under-par 66.

The 23-year-old amateur stunned the crowd at the U.S. Open as he finished tied for fifth in his first major start and he followed that up with a flawless round, featuring four birdies.

Schauffele has company at four under, with Fabian Gomez, Hudson Swafford, Padraig Harrington, Brian Harman, Boo Weekley, Tom Hoge, Matt Every, Scott Piercy, Mark Hubbard, Kevin Na, Daniel Summerhays and Beau Hossler alongside him.

As for Rory McIlroy, the Northern Irishman is a stroke further back following his 67.

McIlroy missed the cut at Erin Hills but he bounced back with four birdies and just the one bogey.

Defending champion Russell Knox started his quest for back-to-back titles by recording a first-round 69.

Day, meanwhile, endured another forgettable round as he ended Thursday two over the card and out of form.

The Australian posted rounds of 79 and 75 to miss the U.S. Open cut at 10 over and his outing in Cromwell was not much better following a 72.