Spieth, who has endured a difficult start to the year, made light work of dangerous Frenchman Victor Dubuisson with a 5&4 success to maintain his position at the summit of Group 1.

The win for the American, who will bid to defend his Masters title when the first major of 2016 is due to get underway on April 7 at Augusta National, came after he had beaten Wales' Jamie Donaldson 3&2.

Northern Ireland's  McIlroy, the current world number three, endured a tough outing against American Smylie Kaufman as he had to come from behind before wrapping up a 3&1 win.

The four-time major champion had got his pursuit of a successful title defence off to a shaky start at the event in Texas by beating Thorbjorn Olesen on the last hole as the Dane bogeyed to hand McIlroy victory.

Day, who came into the WGC-Dell Match Play Championship having triumphed at the Arnold Palmer Invitational on March 17, suffered a back injury in a 3&2 victory over Graeme McDowell of Northern Ireland in his opening encounter in Austin.

The back spasms did not hamper his second match of the tournament, however, as he overcame Thailand's Thongchai Jaidee 5&3 in Group 2.

Jaidee joins the Australian in the next phase of the competition, where the top two players from each of the eight groups will battle it out in a knockout format, after both McDowell and Paul Casey halved their contest, eliminating both the Northern Irishman and his English counterpart.

World number five Rickie Fowler of the United States is staring an early exit in the face after he halved his second match with American Scott Piercy in Group 5.

The result followed a 2&1 defeat to compariot  Jason Dufner in his opener.

The group stage continues tomorrow, with the knockout round scheduled for Saturday (March 26).