The world No 2 — he can overtake Jordan Spieth and regain the No 1 spot if he wins the title for the second time on Sunday — came back from two down after four holes to put McDowell to the sword for the second time in this event.
However, it remains to be seen if the Australian will even play his day two match with Thongchai Jaidee today, with the Thai also unbeaten following his 2 and 1 win over Paul Casey in their Group 2 clash.
Having won the Arnold Palmer Invitational at Bay Hill on Sunday and with two Top-3 Masters finishes since 2011, many see 28-year old Day as one of the clear Masters favourites. His latest injury scare — he was severely hampered by vertigo in last year’s US Open and has suffered frequent injuries during his career — casts a shadow over what looked like a perfectly timed run to Augusta National given his performance at Bay Hill on Sunday.
McDowell felt he let a chance for some matchplay revenge get way away after he holed from five feet for birdie on the third and was conceded the fourth after Day found trouble off the tee to go two up.
But Day didn’t go away and his power was key as he hit a 356 yard drive to the edge of the fifth green and two putted for birdie to reduce the deficit to one hole.
He then birdied the sixth, eighth and ninth to turn one up and doubled his advantage at the par-three 11th where McDowell hit his tee shot into the water and conceded.
He appeared to be cruising to an easy win when he birdied the par-five 12th from 12 feet to go three up.
But he experienced shooting pain in his lower back and legs after his tee shot at the 15th, where he holed a clutch 11 footer for par and watched as McDowell missed from a similar distance for the win.
Day then cried out in pain and dropped his driver on the 16th tee, rubbing his back gingerly before hobbling into a bunker to play his second.
He had to go down on his hunkers and then his knees to relieve the pain but won the match 3 and 2 with a half in par fives with McDowell saving him the bother of bending over to mark his tap in, conceding the putt before missing a 20 footer to extend the match.
Day refused to stop to talk to reporters but the Australian news agency AAP reported him as saying: “It’s really hard to play with searing pain in both sides of your back. I am really struggling as my back has seized up and I need to get back to the clubhouse and for treatment and see what’s wrong.”
The event features 16 groups of four with the leading points scorer after three matches advancing to the last 16 on Saturday, when play will be straight knockout.
There were surprises in some of last night’s early matches with No 58 seed Jason Dufner beating close friend and world No 5 Rickie Fowler 2 and 1.
Phil Mickelson also showed he’s close to his best form with a 5 and 4 thrashing of Matthew Fitzpatrick Sergio Garcia beat Ryder Cup team mate Lee Westwood one up in a match featuring just three halves.
Shane Lowry was all square with Martin Kaymer after nine holes but Rory McIlroy was one down to Dane Thorbjorn Olesen at the turn. Meanwhile Jordan Spieth was 2 up against Jamie Donaldson with four holesto go.
Former Ryder Cup team-mates Sergio Garcia and Lee Westwood endured a rollercoaster contest which saw just three holes halved before Garcia emerged victorious on the 18th.
Garcia was two up after three before losing the next four holes in succession, only to then win four holes in a row himself from the ninth to gain some semblance of control.
“It was very, very funny,” Garcia told Sky Sports.
“We halved just three holes in the whole 18. Lee said to me on 17, ‘We are gelling well!’ It would have been better in a fourball.
“It was very up and down. I started very well and he hit a couple of poor shots, then he got on a great run and the last three or four holes he was making birdie or I was. 2
“I’m very happy with the putt I made on 18 to get the win.”