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Americans Phil Mickelson and Scott Verplank will go into the final day of the Houston Open sharing a one-stroke lead after superb rounds on Saturday.

Three-time Masters champion Mickelson hit top form in the final event before Augusta, making nine birdies in a 63.

Verplank then joined him on 13 under with a third-round 65 at Redstone.

Ireland's Padraig Harrington was close to the leaders until bogeys on the last three holes put him back to nine under.

Harrington was disappointed with his finish but was relieved he still had 18 holes on Sunday to turn things around.

"I got a bit frustrated, he told �鶹Լ�� Radio 5 live. "I felt I was a bit defensive in the middle of the round.

"The course was on for scoring today. I should have been a bit more aggressive. I tried but I ended up playing like a guy leading the tournament on a Sunday - and it's Saturday!"

Englishmen Ross Fisher and Brian Davis are among a group on six under after carding a 69 and a 67 respectively, while compatriot Lee Westwood slipped back to two under with a 74.

Mickelson, who will defend his Masters title next week, matched the course record, and admitted it was a confidence boost ahead of Augusta.

"It doesn't hurt," said the 40-year-old. "I knew that I was close game-wise but I hadn't been putting together the score.

"To get a good round like this means a lot.

"Also to have the challenge of tomorrow, to be in contention, to be in the final group, have an opportunity to win, I really enjoy that opportunity, that challenge.

"I think it's good for me to be in that position heading into next week, too."