Phil Mickelson and five others shared the first-round lead and Rory McElroy was in a group one shot behind at the World Golf Championships-Mexico Championship on Thursday after the first round in Mexico City.
British Open champion Henrik Stenson withdrew after 11 holes with a stomach virus.
With the course at 7,800 feet, the ball was flying. Mickelson blasted a tee shot 379 yards and Roberto Castro smoked one 407 yards, but no one could shoot lower than 4 under.
Only 27 finishers in the 77-player field managed to break par.
Mickelson totaled six birdies but had back-to-back bogeys on Nos. 4 and 5.
"Even though the golf course doesn't play long because of the altitude, it is challenging in many other respects with the precision of the irons, the small targets that the greens present and the speed and undulation of the greens," Mickelson said.
Westwood made eight birdies, but he finished with two bogeys to bring him back to the field. Westwood and Walker each reached 6 under before faltering a bit.
"It's a great golf course," Westwood said. "You've got to be really patient. It's a pleasure to play a golf course where your caddie doesn't hand you the driver walking off the previous green. You've got to put in a bit of thought on this golf course."
McIlroy was back in action for the first time in seven weeks after suffering a rib injury. Other than fighting off the effects of a stomach ailment, he handled his return just fine.
McIlroy's round included an eagle on the par-5 sixth hole.
"I've waited long enough to play," McIlroy said. "I wanted to get out here and be competitive and try to shoot a good score."
Walker was pleased despite his late bogeys.
"Good to post a 4 under, especially on a course ... no one has seen it," Walker said.
Johnson, a winner two weeks ago at Riviera, missed a number of short birdie putts but still managed a 70. Spieth, meanwhile, overcame two early bogeys to finish the day at par.