Rory McIlroy and Danny Willett in a race to be fit for the PGA Championship after sustaining back injuries

Both men have suffered with back problems at the Players Championship

McIlroy and Willett have both been struggling with injury (Getty)

Danny Willett and Rory McIlroy are major doubts for the BMW PGA Championship after suffering from ongoing back problems in the Players Championship, where 54-year-old Vijay Singh was just three shots off the halfway lead.

Willett admitted his body and mind need a rest after being forced to withdraw from the £8.1million event, the former Masters champion retiring after nine holes of the second round.

McIlroy then revealed he would have an MRI scan on Monday to determine the extent of his own back problem, with the world number two admitting the "most important thing" is to be fit for June's US Open.

The BMW PGA Championship takes place in a fortnight's time and has a prize fund of £5.4million as the first event of the European Tour's Rolex Series.

"Hopefully, touch wood," Willett said when asked about his prospects of competing at Wentworth. "We'll see how things go.

"I've already limited the schedule down a lot. Hopefully, all going well, we get back on. We have a week to do some work and see how it feels."

Willett has been struggling for form (Getty)

Willett, hampered by back problems throughout his career, struggled to an opening 79 and covered the back nine at Sawgrass in 40 before calling it a day.

The 29-year-old from Sheffield wrote on Twitter: "Not good to have to withdraw but swinging very poorly is putting a lot of unwanted strain on the back.. body and mind need a rest!!"

Willett has slipped from a career-high of ninth in the world to his current position of 21st after a string of poor results, including becoming the first defending champion since Mike Weir in 2004 to miss the cut at Augusta National.

He also made an early exit from his next event in the RBC Heritage, where he split with long-time caddie Jonathan Smart after the first round.

McIlroy's injury is a recurrence of the back and rib problem which kept him out for seven weeks at the start of the season, although he was able to add a 71 to his opening 73 to finish level par alongside Masters champion Sergio Garcia and world number one Dustin Johnson.

"I hit a couple of tee shots practising on Monday and just didn't feel right and I've been getting treatment on it," McIlroy told Sky Sports. "We just hope it's not the same thing, the joint and the rib.

"It feels more like a muscle strain so I'm going to get an MRI scan on Monday after the tournament and I might have to rest for a week or two. As long as I get myself right for the US Open, that's the most important thing."

McIlroy was just five shots off the clubhouse lead when he finished his round, but ended the day nine adrift of former Open champion Louis Oosthuizen and American Kyle Stanley.

McIlroy is another player to be feeling the strain (Getty)

Oosthuizen added a flawless 66 to his opening 69 and has carded just one bogey in 36 holes, while Stanley also shot 66 with eight birdies and two bogeys.

JB Holmes was two shots off the lead after dropping shots on his last two holes, with Singh another stroke back after a three-putt bogey on the 18th was the only blemish in a 68.

Singh would surpass the 52-year-old Sam Snead as the oldest winner in PGA Tour history with his first victory since 2008.

The former world number one is suing the PGA Tour for "reckless administration and implementation" of its anti-doping program after he was banned for using deer antler spray in 2013.

The ban was subsequently rescinded when the World Anti-Doping Agency said they no longer considered use of the spray to be a doping violation without a positive test, but Singh filed a lawsuit in New York claiming the Tour's actions left him "'humiliated, ashamed, ridiculed, scorned and emotionally distraught."