The American left-hander has endured a rough start to his 2013 campaign, having also battled a sore throat during the US PGA Tour's season-opening Hyundai Tournament of Champions in Maui where he tied for fourth.
"My energy level is not where I want it to be, about 80 per cent right now," Watson said at the TPC Scottsdale in Arizona after partnering former Olympic swimmer Michael Phelps in the pro-am competition.
"I've had about seven days off taking medicine, so I'm still kind of under the weather trying to get the medicine out of me, kind of groggy.
"It's a struggle. My golf game is not where I want it to be.
I've lost about 10 pounds since Maui, but I want to be out there competing. Hopefully I can get some juices flowing and play good this week."
World number nine Watson heads a relatively strong field at the TPC Scottsdale which includes twice champion Phil Mickelson, Germany's former world number one Martin Kaymer and triple Major champion Padraig Harrington.
Also competing this week are world number seven Brandt Snedeker, the FedExCup champion, and 10th-ranked fellow American Jason Dufner.
Irishman Harrington, who like Kaymer is playing in the $6.2 million Phoenix Open for the first time, was in an upbeat mood after producing good putting form in his first two European Tour starts of the year.
"Certainly if the first two weeks are anything to go by, it was in good stead," he said of his fourth place at the Volvo Golf Champions and a tie for 23rd at the Abu Dhabi HSBC Golf Championship.
"If I can play, swing and hit the golf ball like I did last year and putt like I did in those two events, things are looking good for me. I'm quietly confident going forward."
Asked for his initial impressions of the TPC Scottsdale, Harrington replied: "It's a good test of golf. The greens are really nice.
"But it's probably not quite what I expected. I knew I was coming to a desert golf course, and the front nine doesn't quite look like that most of the time. There are quite a lot of trees.
"It kind of plays and looks like a regular tree-lined course but if you do go into that desert, it doesn't seem to stop running into it and finishes in a cactus."
Former world number one Vijay Singh will command more attention than initially expected after admitting on Wednesday to using deer antler spray, which contains an insulin-like growth factor banned under the US PGA Tour's anti-doping programme.
Fijian Singh, a three-times Major winner, has been using the spray for a couple of months but said he was unaware the extract contained a prohibited substance, a natural anabolic hormone that stimulates muscle growth.
Singh is scheduled to tee off in the company of American Ryan Moore and Swede Carl Pettersson from the 10th hole in Thursday's opening round at the TPC Scottsdale.
American Kyle Stanley is back to defend the title he clinched by one shot last year, just one week after he agonisingly squandered a seven-stroke advantage in the final round to lose the Farmers Insurance Open in a play-off.
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