AUSTIN, Texas -- Rory McIlroy had to go 20 holes. Jason Day only played six. Both advanced Friday to the round of 16 in a WGC-Dell Match Play that is loaded with Americans and a Texan who feels right at home.
Jordan Spieth got off to another quick start and sailed into the round of 16 with a 3-and-2 victory over Justin Thomas. The No. 1 player in the world also is the top crowd pleaser at Austin Country Club, where he used to qualify for matches during his brief time wearing the burnt orange of the Texas Longhorns.
The rest of the so-called "Big Three" joined him, even if Day and McIlroy reached the knockout stage in a different manner.
Day was all square in his match when Paul Casey withdrew because of a stomach illness that has been bothering him the last two days. Day was likely to advance, anyway, but the short day at least allowed him extra rest for his back and for the weekend ahead.
Day has played only 31 holes in three days.
"If I can make it all the way through Sunday, that means I'm playing 72 holes on the weekend, which is a lot of golf," Day said.
McIlroy was in one of the four winner-take-all matches between undefeated players in their groups, and it was struggle from the start against Kevin Na. McIlroy, the defending champion, didn't take the lead until the 15th hole, gave it right back and both missed birdie putt on 18 to halve their match.
It ended on No. 2, the second extra hole, when Na hooked his tee shot into a hazard and McIlroy holed a 6-foot putt for the win. He now has 10 straight matches dating to his victory last year at Harding Park in San Francisco.
"I was dying after 10 holes, but I knew I'd played the back nine well," McIlroy said. "It is a sign of relief. It's nice to get to the weekend."
What felt like a marathon over three days now turns into a two-day race to the finish, with two sessions on Saturday and Sunday of single elimination. The round of 16 features 11 Americans, the most since there were 12 out of 16 in 2002.
Phil Mickelson also had a short day and was headed home. Patrick Reed holed out for an eagle on No. 10 to go 7 up, and Mickelson did well to get to the 14th hole before Reed closed him out, 5 and 4. Reed has never trailed in any match all week.
Kirk and Branden Grace had an easy time winning their matches to finish 2-1 in their group. In a peculiar decision this year, players had to wait until everyone was on the course before the playoff. They were the first match and had to wait nearly three hours to start the playoff. It didn't last long, as Grace went from a fairway bunker, over the green, and made bogey.
An, who won the BMW PGA Championship at Wentworth last year, lost a chance to win his group when Rickie Fowler birdied the last hole to halve the match. That sent An into a playoff with Scott Piercy, who won the 18th hole to beat Jason Dufner. An hit his approach to 4 feet on the first playoff hole to advance.
Motoring right along was Zach Johnson, who has been the most dominant player this week in Austin. Johnson finally played the last three holes -- but only for practice. He had an easy time against Shane Lowry, who was eliminated regardless of the outcome, and beat him, 4 and 3.
Johnson has played only 42 holes in three matches, all ending at the 15th or earlier.
"My scorecard, or whatever you want to call it, speaks for itself," Johnson said. "With the exception of the first hole yesterday, I've never been down in a match. I'm not going to take this for granted. It's not going to be easy no matter who my opponent is tomorrow and the rest of the weekend, hopefully. But I'm seeing my shots, my execution is there. I think I'm picking out good targets and I've obviously putting pretty well."
So is Louis Oosthuizen, who has not lost this week and joined McIlroy as the only players to reach the round of 16 two straight years in the round-robin format. Next up for Oosthuizen is Spieth, and he knows what awaits.
"I think it's going to be against 10-15,000 people," Oosthuizen said.
In other groups:
• Rafa Cabrera-Bello of Spain halved his match to advance.