Throughout this year's Masters, we will be keeping a close eye on the top four players in the world rankings - Jason Day, reigning champion Jordan Spieth, Rory McIlroy and Bubba Watson, a two-time winner at Augusta.

Spieth made a superb start to his title defence on Thursday, a bogey-free 66 lifting the 22-year-old top of the leaderboard, the position he held throughout last year's tournament.

Day's round was ruined by a dismal finish, while McIlroy also dropped shots late on after briefly climbing into a share of second and Watson endured a miserable back nine on his way to 75.

JORDAN SPIETH

First-round score: 66 (out in 33, back in 33)

Position: 1st

Spieth could hardly have asked for a better start to his title defence. The reigning champion got hot early, with his approach play and putting particularly strong, and avoided any dropped shots despite finding himself in a couple of perilous positions on the back nine.

Birdies at the third, fifth and eighth lifted last year's wire-to-wire winner into sole possession of the lead and he then picked up another shot on 10.

However, it was not all plain sailing for Spieth, who was forced into superb par saves at the 11th and 16th either side of his fifth gain of the day on 13.

Having worked hard to keep a bogey off his card, the world number two was clinical at the last, holing out for a birdie three after a precise approach had set up another opportunity to improve his score.

RORY MCILROY

First-round score: 70 (out in 34, back in 36) 

Position: T9

In contrast to Day, McIlroy went under the radar early on before bursting into life just as the world number one fell apart.

The Northern Irishman took advantage of Augusta's par fives and was only two off the lead at four under when he followed an eagle at 13 with a birdie at 15.

However, McIlroy then three-putted from an awkward position at the back of the 16th green and was unable to save par at the last after his ball plugged in a greenside bunker. 

JASON DAY

First-round score: 72 (out in 31, back in 41)

Position: T21

An eagle at two, where he put his second shot to 11 feet from 257 yards, set the tone for Day's sensational front nine, which also featured birdies at the fifth, eighth and ninth.

The latter of those gains was almost another eagle as Day's second shot finished inches from the flag, but that was to be as good as it got for the world number one.

After cancelling out a bogey on 10 with a birdie at 13, Day dropped five shots in three holes from the 15th to tumble dramatically down the leaderboard.

Bogeys at 15 and 17 came either side of a shocking six on the short 16th, where he found water off the tee before three-putting for the third time on the back nine.

BUBBA WATSON

First-round score: 75 (out in 34, back in 41)

Position: T54

A player seemingly ideally suited to Augusta, Watson birdied the first and was three under after further gains at the seventh and eighth.

The 2012 and 2014 champion fell away badly thereafter with a flurry of bogeys and appeared extremely low on confidence by the time he, like Day, pulled his tee shot at 16 into the drink.

Wild off the tee throughout the back nine and shaky on the greens, Watson will need a much-improved performance on Friday to put himself in contention.