Accenture World Match Play Championship, Arizona:
Venue: Ritz-Carlton GC Dates: 17-21 February Coverage: Live on Sky Sports, regular updates on BBC Radio 5 Live, scorecards and reports on BBC website
McGowan appearance is down to the absence of Woods and Mickelson
England's Ross McGowan caused an upset by beating top seed Steve Stricker in the first round of the Accenture World Match Play Championship in Arizona.
World number 66 McGowan, who was a late inclusion, beat the American with a 25-foot birdie putt on the 19th hole.
Northern Ireland's Rory McIlroy staged a magnificent comeback to defeat American Kevin Na on the final green.
And there were wins for England's Lee Westwood, Paul Casey, Ian Poulter, Luke Donald and Oliver Wilson.
McGowan is only in the tournament because of the withdrawals of world number one Tiger Woods and world number three Phil Mickelson.
"I would say I fancied my chances - and I could go all the way," said the Essex-born 27-year-old, who faces Japanese teenager Ryo Ishikawa in Thursday's second round.
"I had a shaky start but then played really nicely and it was quite exciting when that one dropped on 19."
McGowan was little known until he finished runner-up to Westwood at the Dubai World Championship in November and was a massive underdog against Stricker.
He becomes only the second 64th seed to beat the top seed in the history of the event. Australian Peter O'Malley eliminated Tiger Woods in 2002 to become the first.
"History is showing in this event," added Stricker. "It can happen to anybody and that's just the nature of it.
"It's kind of scary because you don't know what you're going to run into. He started putting it really good coming in and scared the hole a bunch of times."
McGowan made the impressive putt on the first extra hole to oust Stricker, who climbed to number two in the world rankings with victory at the recent Northern Trust Open in Los Angeles.
But this defeat means he could be replaced in that position by Westwood - if the 36-year-old from Worksop wins the title.
"I'm feeling good," stated Westwood, who started with a 3&1 win against fellow Englishman Chris Wood. "I've lost a bit of weight and am getting a lot stronger - and this is the ideal week to test it all out.
"I have a little bit of a problem in the match play events because 18 holes is just a short period of time and you really have to be fast off the mark.
"You've got to understand in your own mind that you're going to have close matches."
The 20-year-old McIlroy, the fifth seed, was four down early on and two behind with four to play but managed to secure a 1-up win over his unheralded opponent Na.
McIlroy plays world number 40 Wilson in the next round and said: "I hung in well but was pretty fortunate to get through and I'll need to play better if I want to progress into the latter stages.
"Ollie's a great competitor, a Ryder Cup player and he's going to be very tough."
Donald carded eight birdies to see off former Walker Cup team-mate Graeme McDowell 2&1.
I'm feeling good. I've lost a bit a weight and getting a lot stronger - and this is the ideal week to test it all out
World number 23 Donald was runner-up in Los Angeles two weeks ago and looks back to his best, while McDowell is now in danger of dropping out of the world's top 50.
"The last two years I've run into a buzz saw," said McDowell. "Last year Zach Johnson made eight or nine birdies and Luke just putted the eyes out of it. Every time I smelt a chance he made a putt.
"It does not matter how well you play in this. If someone plays better you go home."
Poulter, too, staged a fine comeback as he went from two up to two down against former Open champion Justin Leonard before winning on the first extra hole.
Casey, last year's runner-up to Australian Geoff Ogilvy, had a thumping 5&4 success over Canadian Stephen Ames, while Wilson defeated Spaniard Miguel Angel Jimenez 3&2.
Out, though, went Volvo World Match Play champion Ross Fisher of England, who was crushed 5&4 by Thai Thongchai Jaidee, as well as three-time major winner Padraig Harrington, who surprisingly lost 3&1 to Indian Jeev Milkha Singh.
Another Englishman, world number 54 Simon Dyson, was beaten 3&1 by American Sean O'Hair.
Ogilvy romped to a 7&5 victory over Swede Alexander Noren, Spain's Sergio Garcia had a 2-up win over David Toms and Dane Anders Hansen had a notable 2&1 win over US Open champion Lucas Glover.
Open champion Stewart Cink matched McIlroy's fightback when he also turned round a four-hole deficit to beat Italian Edoardo Molinari on the last - and Molinari's brother Francesco lost on the 21st to Zach Johnson.
Former winner and world number 10 Henrik Stenson withdrew after only one hole of his match against American Ben Crane because he was suffering from flu symptoms.
FIRST ROUND RESULTS:
(USA unless stated, seeded positions in brackets)
(12) Robert Allenby (Aus) bt (53) Peter Hanson (Swe) 4 & 2
(21) Luke Donald (Gbr) bt (44) Graeme McDowell (Gbr) 2 & 1
(11) Kenny Perry lost to (54) Brian Gay 2 & 1
(22) Zach Johnson bt (43) Francesco Molinari (Ita) at 21st
(9) Ian Poulter (Gbr) bt (56) Justin Leonard at 19th
(24) Angel Cabrera (Arg) lost to (41) Adam Scott (Aus) 3 & 2
(10) Geoff Ogilvy (Aus) bt (55) Alexander Noren (Swe) 7 & 5
(23) Camilo Villegas (Col) bt (42) Dustin Johnson (12) 4 & 3
(5) Rory McIlroy (Gbr) bt (60) Kevin Na 1 hole
(28) Miguel Angel Jimenez (Spa) lost to (37) Oliver Wilson (Gbr) 3 & 2
(6) Paul Casey (Gbr) bt (59) Stephen Ames (Can) 5 & 4
(27) Alvaro Quiros (Spa) lost to (38) Mike Weir (Can) 8 & 6
(8) Padraig Harrington (Irl) lost to (57) Jeev Milkha Singh (Ind) 3 & 1
(25) Anthony Kim lost to (40) Matt Kuchar 3 & 2
(7) Henrik Stenson (Swe) lost to (58) Ben Crane (USA) 1 down (conceded)
(26) YE Yang (Kor) bt (39) Soren Kjeldsen (Den) 2 & 1
(13) Sergio Garcia (Spa) bt (52) David Toms 2 holes
(20) Lucas Glover lost to (45) Anders Hansen (Den) 2 & 1
(14) Sean O'Hair bt (51) Simon Dyson (Gbr) 3 & 1
(19) Stewart Cink bt (46) Edoardo Molinari (Ita) 2 holes
(16) Robert Karlsson (Swe) bt (49) Rory Sabbatini (Rsa) at 20th
(17) Ross Fisher (Gbr) lost to (48) Thongchai Jaidee (Tha) 5 & 4
(15) Ernie Els (Rsa) bt (50) Ryan Moore 4 & 3
(18) Retief Goosen (Rsa) bt (47) Soren Hansen (Den) (14) 3 & 2
(4) Martin Kaymer (Ger) bt (61) Chad Campbell 4 & 2
(29) Vijay Singh (Fij) lost to (36) Tim Clark (Rsa) 1 hole
(3) Jim Furyk bt (62) Scott Verplank 2 & 1
(30) Hunter Mahan lost to (35) Charl Schwartzel (Rsa) 2 & 1
(1) Steve Stricker lost to (64) Ross McGowan (Gbr) at 19th
(32) Ryo Ishikawa (Jpn) bt (33) Michael Sim (Aus) 2 holes
(2) Lee Westwood (Gbr) bt (63) Chris Wood (Gbr) 3 & 1
(31) Nick Watney bt (34) Yuta Ikeda (Jpn) 4 & 3