Mickelson was able to claim the improbable victory only after Frank Lickliter three-putted from six feet for triple-bogey at the third extra hole at Torrey Pines.
Mickelson, who won for the 18th time in his career, exchanged positions with Ernie Els, who dropped to the third spot after spending last week on vacation in his native South Africa. Els has committed to play in the Southern Africa Tour's season-ending Tour Championship from February 22-25.
Top-ranked Tiger Woods recorded his third top-10 showing in four starts this season when he finished alone in fourth at the Buick Invitational. He has now gone seven consecutive starts without a win, matching his longest victory drought since 1999.
David Duval and Lee Westwood, who were both idle this week, remained in the fourth and fifth spots, respectively. Davis Love III stayed in sixth place after bowing out of the Buick Invitational playoff on the second hole of sudden death.
The next eight players held their positions from last week. Colin Montgomerie, who will make his 2001 debut at the inaugural Singapore Masters next week, held seventh place, while Vijay Singh, Darren Clarke and Hal Sutton rounded out the top-10.
Mercedes Championships winner Jim Furyk was 11th, followed by Tom Lehman, Nick Price and Jesper Parnevik.
Spain's Sergio Garcia was 15th after switching places with Michael Campbell of New Zealand. Garcia lost to 19-year-old Australian Aaron Baddeley in a playoff at the Greg Norman Holden International on Sunday, while Campbell, the winner of the Heineken Classic the week before, missed the cut at the Holden event.
Stewart Cink remained in 17th place, followed by Canadian lefthander Mike Weir in the 18th spot and Denmark's Thomas Bjorn in 19th. Weir traded places with Bjorn after notching his second straight top-10 finish of the season, a tie for fifth place at the Buick Invitational.
John Huston, who was 21st in the World Rankings last week, moved up one spot to No. 20, replacing Loren Roberts. Roberts missed the cut at Torrey Pines.
Lickliter's role in the Buick playoff helped him vault eight spots, from No. 87 to No. 79.
Baddeley's victory Sunday was his second as a professional and allowed him to crack the top-100 for the first time at No. 99.
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