"I didn't strike the ball particularly well at Hilton Head a couple of weeks ago, so I put in some work at the end of the week with my instructor at home,'' Spieth said. "It was a solid round, one bogey and I think (seven) birdies. I would certainly take it the rest of the week.''
Spieth, ranked second in the world, put himself in a stronger position to advance to the knockout stage of the tournament that begins Saturday.
The top player in each of 16 groups makes it through after three days of round-robin play.
Among those who were defeated Wednesday were: Henrik Stenson, ranked third in the world and a winner of this tournament in 2007; sixth-ranked Justin Rose, who just won the Zurich Classic on Sunday; defending champion Jason Day; and Ian Poulter, the Ryder Cup match play stalwart who won this event in 2010.
Under the old WGC-Match Play format, all would have been going home. But a new format was instituted this year which has 32 matches for the first three days in 16 groups. All of those who lose will now need help, however, to advance to the weekend.
Spieth was 3-up through eight holes but lost the ninth and 11th to see his lead shrink to one hole.
But he then birdied the next two holes to go 3-up again and closed out the match on the 16th.
"It's good to get off to a good start,'' he said. "I played solid golf today. It got a little interesting there on the back nine. Big turn around on 12 and 13 to get 3-up. I'm extremely pleased.''
Ilonen, who played in the China Open last week on the European Tour, flew to San Francisco not even knowing if he would get in the field.
He got the last spot when Phil Mickelson withdrew. Ilonen will face Westwood on Thursday.