Day, a former world number one, and Todd shot five under par at TPC Harding Park.

The pair are one shot clear of a group of nine players, including Brooks Koepka of the US, who is bidding for a third straight PGA Championship title.

"I feel like the momentum that I've had over the last three starts has kind of seeped into this week," said Day, who has recorded two top-10 finishes in the past two tournaments. 

"I've been working very hard in the off-weeks to get my putting back to where it is. 

"I feel like the game and the confidence is slowly coming around."

Olympic champion Justin Rose, American Zach Johnson and Germany's Martin Kaymer are among the others to have carded a four-under-par 66 in the opening round of the season's first major.

Gary Woodland of the US and England's Tom Lewis are a stroke further behind on three under par.

Fifteen-time major winner Tiger Woods shot his lowest opening round in a major since 2012 with a two-under-par 68.

New world number one Justin Thomas endured a difficult round, carding a one-over-par 71 to end the first day six shots off the lead.

The tournament is taking place without spectators because of the coronavirus pandemic.

The PGA Championship is the first major of the season for the first time since 1971 after the COVID-19 crisis forced a reshuffle of the 2020 calendar.