Søren Kjeldsen opened up a narrow lead over World Number Two Rory McIlroy as the WGC - Dell Technologies Match Play got under way at Austin Country Club.
The four-time European Tour winner was playing just his second event of the season but showed no signs of rust, making two birdies on the front nine to open up a one up lead over the 2015 champion.
Kjeldsen put his approach to the first to six feet to quickly go one up with a birdie but McIlroy hit straight back on the third with the help of a huge drive and a 13-foot putt.
A tee-shot to 16 feet on the par three fourth moved the Northern Irishman one up but Kjeldsen was displaying some wonderful iron play and an approach to ten feet on the fifth brought a second birdie of the day.
A McIlroy bogey on the ninth then handed the Dane a one-shot lead at the turn.
In the other Group 2 clash, Gary Woodland and Emiliano Grillo were all square after eight holes.
In Group 15, Branden Grace was two up against Andy Sullivan in a real see-saw battle.
Sullivan was ragged off the tee on the third to send Grace one up but his tee-shot on the fourth was to within ten feet to even things up.
Grace hit an approach close on the fourth to edge back ahead and then holed a brilliant bunker shot on the seventh to establish a cushion before Sullivan hit back with a birdie on the next.
An approach to six feet then re-established the South African's advantage.
In the other Group 15 encounter, Brandt Snedeker and William McGirt were all square after ten holes.
