Ross Fisher held the lead midway through the second round of the WGC – Mexico Championship but Rory McIlroy was hot on his heels.
Fisher started his round from the 10th and picked up shots on the 11th, 15th and 16th before missing the green at the short 17th and bogeying.
However, the former Ryder Cup player birdied the next from close range to spearhead a strong European challenge, with McIlroy just a shot behind on his return to competitive action.
McIlroy had played just once previously in 2017 due to a rib injury which saw him push through the pain barrier before losing to Graeme Storm in a play-off for the South African Open hosted by the City of Ekurhuleni in January.
The 27-year-old still arrived in Mexico knowing that he could reclaim top spot in the Official World Golf Ranking for the first time since August 2015 by winning his third WGC title, as long as current number one Dustin Johnson finishes joint fourth or worse.
McIlroy started his second round by driving the green on the short par-four first before three-putting for par, but then birdied the second, fourth and sixth to reach six under.
On a highly-congested leaderboard, England's Andy Sullivan and Tyrrell Hatton were alongside Mickelson, Jimmy Walker and Roberto Castro on five under, with Lee Westwood and Johnson a shot further back.
Olympic champion Justin Rose was on three under alongside Ryder Cup team-mates Sergio Garcia and Martin Kaymer, with 31 of the 76-strong field separated by just five shots.
