England's Matt Fitzpatrick put himself on course for a hat-trick of Omega European Masters titles with a third-round 67.
The former U.S. Open Championship winner, who turned 29 on Friday, combined seven birdies with two bogeys and a double bogey on the 13th to move to 15 under par.
Fitzpatrick, who became the first player since Seve Ballesteros to defend a title at Crans-sur-Sierre with his victories in 2017 and 2018, is two clear of Swedes Ludvig Aberg and Alexander Björk and Scotland's Connor Syme.
Fitzpatrick began the day one ahead of Björk but missed the first fairway and was left short-sided with his approach, leading to an opening bogey.
He responded from 20 feet for birdie on the second and a fine tee shot to six feet on the third was followed by a pitch to three feet on the fifth.
Another birdie on the seventh after a pitch to four feet left Fitzpatrick threatening to run away with it but he experienced a mid-round wobble.
A further gain looked to be on the cards at the ninth when an imaginative chip using the backstop travelled round the green and returned to four feet.
However, his birdie putt broke sharply and Fitzpatrick pushed the return effort wide.
A pulled tee shot on the 13th found water leading to a double bogey which briefly saw Björk hit the front but Fitzpatrick responded with three birdies in his last five holes, including back-to-back gains on the par-five 14th and 15th.
“It was good, sloppy mistake on nine, first putt moved tonnes left to right then second one didn't move,” he said. “That was annoying. Very poor shot on 13 but very happy to play the last five in three under just to get back in it.
“I knew that 14 was reachable in two, 15 is gettable today with the down wind and I was hoping I could pick up one in the last three and did on 18.
“I was really pleased to make sure I was in the mix come Sunday because it could have unravelled if you make two pars on the easy par fives and you can be quite a few behind. Important to get a lead going into tomorrow and looking forward to playing again.”
As well as a third title in the Swiss mountains, which would see Fitzpatrick draw level with Ballesteros for most wins in the event, he will also seal an automatic Ryder Cup berth via the World Points list by finishing in at least a tie for seventh with one other player.
“I feel like there's a lot of things for me this week,” Fitzpatrick added. “I can wrap up Ryder Cup, I can match Seve's record and win here three times. A win here is always special and my third would be even better.”
Björk, whose previous DP World Tour title came in China five years ago, made four birdies and two bogeys in his 68, while compatriot Aberg finished with a hat-trick of gains in his 66.
Syme also made a fast finish with birdies on the 14th, 15th and 16th in a round of 65, with Alex Fitzpatrick and Nicolai Højgaard tied for fifth on 12 under.
