British Open 2015: Jordan Spieth warms up with career-best 61

    Jordan Spieth is in superb form ahead of his challenge for the British Open crown in Scotland.

    (CNN)Halfway to a calendar grand slam, Jordan Spieth warmed up for the British Open at St Andrews with a career-best 61 at the John Deere Classic and went on to win the PGA Tour event.

    The 21-year-old Texan, chasing his third major of the season next week, looks in unstoppable form after his 10-under-par on the TPC Deere Run in Illinois.
    It left Spieth with a two-shot lead over New Zealand's Danny Lee going into Sunday's last round and he duly completed the victory.
      The world number two had to fight hard though. A three-under 68 left him tied with Tom Gillis on 20-under and he sealed his fourth win of the season at the second extra playoff hole.
      Saturday's third round was the standout performance of a week which started with a lackluster level-par 71 and sent out an ominous message to his rivals after claiming the U.S Masters and U.S. Open titles already this year.
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      "Ultimately, this was a big day for me in my preparation for next week," he said.
      Spieth would be the first golfer since the legendary Ben Hogan in 1953 to win the first three majors of the season and his chances have been improved by the withdrawal of arch-rival Rory McIlroy, who has ruptured an ankle ligament.
      Spieth will also likely soon take over at the top of the world rankings from McIlroy with the Northern Irishman sidelined.
      For now, the American is happy to have achieved a personal best PGA round, finishing birdie-eagle-birdie -- including holing out from 105 yards on the par-five 17th.
      It left him with the chance of his best and he revealed a conversation he had with his caddie Michael Greller after his second shot to the 18th to set up a birdie chance.
      "I said 'Mike, I just saw the board and think this is for my lowest round on the PGA TOUR," Spieth told the official Tour website.
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      "He said, 'It doesn't matter. Just keep on trekking'. I said, 'Yeah, but I appreciate this and I really want to make this thing."'
      Meanwhile, Spieth's Ryder Cup teammate Rickie Fowler completed his preparations for the British Open in fine style as he birdied the final hole to win the Scottish Open at Gullane.
      The world number nine looked to have blown his chances when he bogeyed the 14th at the seaside links, but he charged back with birdies at the 15th, 16th and 18th to finish 12 under after a 69,
      It left Fowler, who won the Players Championship at Sawgrass in May, a shot clear of fellow American Matt Kuchar and Raphael Jacquelin of France.