San Francisco, CA (SportsNetwork.com) - Rory McIlroy and Jordan Spieth, the top-two players in the world, picked up easy wins on Wednesday at the WGC- Cadillac Match Play Championship.

McIlroy carded just one birdie, but it was more than enough to beat Jason Dufner, 5 & 4, at TPC Harding Park. Spieth posted seven birdies in 16 holes as he took down Mikko Ilonen, 4 & 2.

Henrik Stenson, Justin Rose and defending champion Jason Day were the highest ranked players to lose on day one. John Senden birdied the 18th hole to square his match with Stenson, then birdied the first extra hole to win 1-up in 19 holes.

Marc Leishman won two of the first three holes against Rose and led throughout. The Australian downed Rose, 3 & 2. Day and Charley Hoffman were tight through 11 holes with one win apiece to go with nine halves. Hoffman birdied three of four holes from the 12th en route to a 4 & 3 win.

The event changed formats this year, with 16 groups of four players playing a round-robin format the first three days, as opposed to the old setup of four groups of 16 in a single-elimination format. The winners of each group will advance to Saturday's round of 16.

McIlroy and Dufner both birdied the first. Dufner stumbled to a bogey at the third. McIlroy ran off eight straight pars from the second, and made the turn with a 3-up lead as Dufner also bogeyed eight and nine.

Dufner's struggles continued as he dropped another shot at 10 to fall 4-down. After three straight pars from both players, Dufner carded his fifth bogey of the day at the 14th to lose the match.

"I think from tee to green I played really solid. I didn't really hole anything. I made one birdie on the first hole and made 13 pars after that," said McIlroy. "Jason didn't have his best day, but this is match play and you just need to beat the guy in front of you, and I was able to do that today."

In the other match in McIlroy's group, Billy Horschel birdied six of 14 holes as he rolled past Brandt Snedeker, 5 & 4.

Spieth birdied the first, but tripped to a bogey at the third to square the match. The Masters champion birdied three of four holes from the fifth to move 3-up.

Birdies at nine and 11 by Ilonen narrowed the gap, but Spieth birdied 12 and 13 to regain a 3-up lead. Spieth also birdied the 16th to close out the match.

"I didn't strike the ball particularly well at Hilton Head and put in some good work at the end of last week with instructor at home. Put it to test out here and it was a good, solid round," Spieth stated. "One bogey and (seven) birdies. I'd certainly take it the rest of the week."

In the other match in Spieth's bracket, Lee Westwood and Matt Every had a back-and-forth battle in which they combined to win 13 of the 18 holes and neither player led by more than 1-up.

Westwood was 1-down after 14, but birdied 15 and 16 to take a 1-up lead. He tripped to a bogey at 17 to square the match, then birdied the last to claim a 1-up win.

Senden birdied the first, but bogeyed the second. He and Stenson traded wins at the seventh and ninth. Stenson birdied the 15th to move 1-up, but Senden birdied the par-5 18th to force extra holes. Senden then birdied the par-5 first to topple the 2007 champion.

Bill Haas never trailed en route to beating Brendon Todd, 3 & 2, in the other match in that group.

Bubba Watson and Louis Oosthuizen easily won their matches. Watson lost just one hole in beating Miguel Angel Jimenez 5 & 4. Oosthuizen cruised to a 6 & 5 win over Keegan Bradley.

Jim Furyk and Martin Kaymer won their matches in group five. Furyk bested George Coetzee, 3 & 2, while Kaymer picked up a 3 & 1 win over Thongchai Jaidee.

Anirban Lahiri beat Ryan Palmer in the other match in Rose's group. The other match in Day's group saw Zach Johnson birdie four of the last eight holes en route to beating Branden Grace, 2-up.

Ben Martin used a hole-in-one on the par-3 17th to take down 2013 champion Matt Kuchar, 1-up. Hunter Mahan, the 2012 winner, rolled to the biggest win of the day as he routed Stephen Gallacher, 7 & 6. The only other past champ in the field is 2010 victor Ian Poulter. The Englishman dropped a 3 & 2 decision to Webb Simpson.

Paul Casey, the 2009 and 2010 runner-up, outlasted Chris Kirk in the longest match of the day. Casey made par on the 22nd hole to win the match.

Charl Schwartzel, Francesco Molinari and Hideki Matsuyama all won their matches by 5 & 4 margins, Gary Woodland and Joost Luiten needed 19 holes to win their day one matches.

NOTES: In the 32 matches on Wednesday, only 18 were won by the higher-seeded player ... Martin's hole-in-one was the first in this event since Ben Crane had an ace in 2006 ... Last year's runner-up Victor Dubuisson was one of the players that lost 5 & 4 as he fell to Schwartzel, the 2011 Masters winner.