Mickelson has been suffering from ill health of late

The world number two has struggled with shortness of breath and fatigue since October and was told by his doctor on Monday to rest for two days.

But Mickelson, 37, said he was "feeling much, much better", and fit to play.

The event will also be Tiger Woods's first of 2008, as he aims to win the title for the fourth successive year.

World number one Woods has won the tournament four times and heads to the Californian course less than two weeks after Golf Channel commentator Kelly Tilghman caused a storm by joking his rivals should "lynch him in a back alley" to curb his domination of the sport.

Woods has since said he has forgiven Tilghman, who was suspended for two weeks for making the remark.

"It was unfortunate. Kelly and I did speak. There was no ill intent. She regrets saying it," said Woods. "In my eyes, it's all said and done."

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Mickelson, a three-times winner at Torrey Pines Golf Course, is scheduled to have a practice session with his swing coach Butch Harmon after Wednesday's pro-am competition.

"I'm happy about that and looking forward to working with Butch (Harmon) to make sure everything's in order for this week," he said.

Of his condition, Mickelson's manager T.R. Reinman said: "It's unclear whether it was caused by working in the smoky debris on his property following the wildfires that devastated Mickelson's neighbourhood or the illness he developed immediately after that in Singapore."

"He managed to win the HSBC Champions the following week in Shanghai but has not felt 100% since then."

Mickelson, who won the Buick Invitational in 1993, 2000 and 2001, has not played tournament golf since clinching the HSBC Champions in a three-way play-off two months ago.