American Johnson, the top seed and world number one, was beaten 3&1 in Group One by Austria's 52nd seed Bernd Wiesberger at the Austin Country Club in Texas.

The 32-year-old has a best finish of 15th at a Major Championships but played some excellent golf to oust Johnson, the US Open champion in 2016.

"I still didn't play great but somehow managed to get it done and I'm just happy with that," said Wiesberger. 

"I thought it was the easiest game I could have.

"I'm the underdog playing with the defending champion."

Northern Ireland's McIlroy, a four-time major winner, was in fine form prior to the event after winning last week's Arnold Palmer Invitational.

However, the number six seed slipped to a 2&1 loss to home American player Peter Uihlein in Group Six.

Triple major winner Jordan Speith had no such trouble as he beat South Africa's Charl Schwartzel, a former winner of the Masters, 2&1 in Group Four.

Reigning Masters champion Sergio Garcia of Spain claimed a one up victory over Shubhankar Sharma of India in Group Seven and Australia's former world number one Jason Day ousted American James Hahn 4&2 in Group Eight.

The 16 group winners will all advance to the knock-out phase, which is due to begin on Saturday (March 24).

This is the second WGC event following on from American Phil Mickelson's victory at the WGC Mexico Championship last month.