"He is an incredible player and talent and has one of the most impressive records, if not the most impressive, in the history of the game.
"I won't ever underestimate him. Regardless of what he did last week, knowing the competitor he is I expect him to come back and be the Tiger that we are used to seeing on the course."
Mickelson has spent 244 weeks of his career at No2 and added: "Being No1 is certainly something I've been striving for so it would mean a lot to me.
"However, for me to accomplish that, I can't focus on that. I've got to go out and get ready to play this golf course because it's not an easy challenge. I've got a lot of work ahead."
Woods, who has confirmed he will be at St Andrews for The Open in July, has had a stranglehold over the No1 spot for the last 258 weeks and has spent 11-and-a-half years in total at the top.
The 14-time major champion insists he is unfazed by the prospect of being dethroned and said: "I've had it happen before. The whole idea to be No1 and continue being No1, you have to win tournaments and I haven't done that in a while."
Woods sounded terribly down after his second practice session although his mood appeared to have more to do with his private life rather than his golfing struggles.
He said: "I've had two different low moments — obviously what I'm going through now and then my father's death."
One of Woods's partners tomorrow will be Ian Poulter, fit again after missing last week's event after hurting his knee playing basketball.
The Englishman said today: "Practice went well and the knee felt great. I played the front nine and am really happy with how I'm playing."
