The family of Australian golf star Jason Day has been involved in a car accident in Boston - but no one has been injured.

The world No.1 carded a one-under-par 70 at the US PGA Tour's second FedEx Cup playoff event before learning of the incident.

Day made a hasty departure from the Deutsche Bank Champioship but officials say he has decided not to withdraw after earlier fears that he may have to pull out.

"After the completion of my round today, I was informed my wife Ellie our children Dash and Lucy and friend Katie were involved in a traffic incident near our RV," Day said in a statement released to AAP.

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"Their car was hit by a bus and Ellie was taken to the hospital for precautionary measures. "Thankfully everyone is now resting comfortably and Ellie has been released.

"They will all be monitored in the coming days.

"We want to thank the emergency services and hospital personnel for their assistance."

Day was sitting in second spot in the season-long points race as the world's premier golfers chase the $US10 million bonus jackpot.

Day's off-course issues mirrored his rollercoaster round.

After starting on the 10th hole, he had three bogeys, two two birdies and an eagle on the par-5 second to be four shots off the pace midway through the opening day.

Englishman Paul Casey and Argentine Fabian Gomez share the clubhouse lead after shooting five-under 66s.

Adam Scott was four under and in a tie for third approaching the turn, while fellow Australian Marc Leishman signed off for a three-under 68.

- AAP