Thomas can replace compatriot Dustin Johnson, whose elimination at the group phase was confirmed yesterday, as world number one if he wins the event at the Austin Country Club.

The 24-year-old was in superb form as he completed the first round of the tournament with a 100 per cent record, thrashing Italy's Francesco Molinari 7&5 to earn a third consecutive victory.

Thomas' feat of winning all three of his group matches was replicated by several other players competing at the prestigious event, including Spaniard Sergio Garcia and American Patrick Reed.

Garcia topped group seven after he recorded a 3&1 success against Xander Schauffele.

Reed, tied for second place at last year's PGA Championship, booked his place in the knock-out stage at the expense of Ryder Cup team-mate Jordan Spieth with a 2&1 triumph in group four.

The 16 group winners all marched on to the next round but the list of the qualified players did not include McIlroy, the Northern Irishman who returned to form by clinching the Arnold Palmer Invitational title.

The world number seven had given himself hope of prolonging his stay at the tournament when he beat Jhonattan Vegas of Venezuela yesterday.

But he was unable to build on that performance as he lost 5&3 to American Brian Harman, who finished in pole position in group six.

The last 16 matches and quarter-finals are due to be held tomorrow.