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Jason Day started the WGC Invitational in Tennessee poorly but is leading a $US1-million promotional series with Dustin Johnson and Brooks Koepka.

Jason Day faces a potential million-dollar showdown with big guns Brooks Koepka and Dustin Johnson, despite making a sloppy start to the World Golf Championships Invitational in Tennessee.

Former world No.1 Day has left himself work to do after an opening two-over-par 72 at TPC Southwind, but his only birdie came at a hole that could deliver him a $US1 million ($A1.5 million) bonus.

Day is tied atop the leaderboard of a season-long promotion called the Aon Risk Reward Challenge.

The Challenge sees players record their best two scores on a designated hole at each tournament throughout the season.

An average to par is calculated and the player with the best score at the conclusion of the season is awarded $US1 million.

Players must have played 40 rounds at the eligible events to qualify, a feat Day, Koepka and Johnson have achieved.

All three are tied at the top, with an average of one-under par.

However, none of them have committed to next week's Wyndham Championship, meaning this is their last chance to improve their average - unless they commit to the Wyndham by Friday afternoon.

Ian Poulter, Jason Kokrak, Justin Thomas, Alex Noren, Gary Woodland and Australia's Cameron Smith all have mathematical chances of winning, but the odds suggest a three-man showdown between Day, Koepka and Johnson.

The designated hole at TPC Southwind is the par-5 16th, with Day and playing partner Koepka each making birdie on Thursday.

"It was my only birdie of the day. It added a bit of excitement to an otherwise disappointing day," Day told AAP.

Day admitted he playfully sledged world No.1 Koepka - one of the longest hitters on tour - by attempting to convince him not to go for the par 5 in two.

"I tried to get him to hit a 3-iron off the tee," Day joked. "Brooks and I are really good friends, but I think it was the first time I have ever rooted against him to make a putt."

However, Day's poor round left him a distant 10 shots back of first round leader Jon Rahm (62) at the no-cut WGC Invitational.

Day said improving his driver was key for the rest of the tournament having hit just six of 14 fairways on Thursday.

"I need to get myself in the fairway; I missed too many today and, if you do that, you're going to shoot over par."