* Day beats Johnson, Sterne wins against Dufner
* McIlroy and Woods among the late starters (Updates with later matches)
MARANA, Arizona, Feb 21 (Reuters) - PGA Tour rookie Russell Henley beat in-form South African Charl Schwartzel 1up to pull off the biggest early upset in the snow-delayed first round of the WGC-Accenture Match Play Championship on Thursday.
The 23-year-old American, who clinched his maiden victory on the U.S. circuit at last month's Sony Open, never trailed and held off a late fightback by the 2011 Masters champion with pars on the last three holes.
Among the late starters, Northern Ireland's world number one Rory McIlroy was 1up on Irishman Shane Lowry after six holes, and three-times winner Tiger Woods was all square with fellow American Charles Howell III after five.
Australian Jason Day, Belgian Nicholas Colsaerts and South African Richard Sterne also delivered upset wins at Dove Mountain on Thursday but Henley's triumph came as the biggest surprise given his opponent's scintillating recent form.
Schwartzel had won twice, sealed two runner-up spots and added three more top-five finishes in his last seven starts worldwide but he struggled against Henley with four bogeys in the first 13 holes.
"I knew Charl was going to be tough," Henley told reporters. "I just give it everything I had, tried to hang tough and play until the end.
"I tried to not get too up or down. I've heard a lot of comments from people saying they expected him to win, and that gave me a little motivation for sure."
Henley will next meet Day, who completed a crushing 6&5 win over Zach Johnson when the American missed a birdie putt from 14 feet at the 13th in a bid to keep their match alive.
Day had been in full command after making four birdies in the first eight holes and he was 6up overnight before Johnson won the par-five 11th with a birdie.
"Everything was good," said Day, who was five under par after 13 holes. "I missed two greens out of the 13 holes that I played, I hit a lot of good quality shots, and I putted great."
Long-hitting Belgian Colsaerts upset American Bill Haas 5&4 and Sterne, also a power hitter, outgunned American Jason Dufner 1up.
In the second round, Colsaerts will take on British world number six Justin Rose who won 2&1 against South Korean veteran K.J. Choi.
Sterne, who had been ousted on the opening round in his two previous appearances at the event, will next meet defending champion Hunter Mahan, who crushed Italy's Matteo Manassero 5&4.
In other matches, American Bo Van Pelt hammered Australian John Senden 6&5, Masters champion Bubba Watson beat Briton Chris Wood 2&1 and Spaniard Sergio Garcia scraped past Thailand's Thongchai Jaidee after 20 holes.
Ian Poulter, the 2010 champion, avoided a third consecutive exit at the first hurdle with a 2&1 victory over fellow Briton Stephen Gallacher and will face Van Pelt in the second round.
The first-round matches finally got under way at Dove Mountain's Ritz-Carlton Golf Club after the scheduled resumption had been pushed back four-and-a-half hours due to unplayable conditions.
Two more inches of snow blanketed the course overnight, lifting the total to four inches in under 24 hours, and hopes for a quick melt were dashed by plunging temperatures and early morning frost.
Just over three-and-a-half hours of golf was possible on Wednesday because of driving rain, sleet and then snow after 23 of the 32 matches had started with none finishing. (Reporting by Mark Lamport-Stokes; Editing by Gene Cherry)