Jordan Spieth has backed Rory McIlroy to increase the pressure on Dustin Johnson when the 97th US PGA Championship resumes at Whistling Straits.
Paired together for the fifth time in their careers, Spieth and McIlroy both carded opening rounds of 71 in tougher afternoon conditions to lie five shots behind Johnson, who took advantage of calmer morning weather to shoot an impressive 66.
McIlroy admitted he was nervous at the start of his first competitive round since June 21 following a seven-week injury lay-off, but Spieth saw enough in the world number one's performance to suggest he could retain his title and claim a fifth major.
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"I didn't see any difference in his game," said Masters and US Open champion Spieth, who is looking to become the third player after Ben Hogan and Tiger Woods to win three majors in one season.
"He seems a hundred per cent ready. Everything seemed to be on point and I expect him to move up the board."
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McIlroy's only concern regarding his left ankle was not getting the strapping wet as he played his fourth shot on the fifth from shallow water on the edge of a hazard, the 26-year-old rolling up a trouser leg before splashing out to 12 feet and saving par.
"It was very important to not go out there and shoot myself out of it right away," said McIlroy, who carded four birdies and three bogeys.
"I wish I could have been one or two shots better, but hopefully I will go out there on the back nine on Friday, where there are some chances, and get off to a good start."
The last time the US PGA was held at Whistling Straits in 2010, Johnson was one shot ahead with one to play and thought his bogey on the 18th meant he would be heading into a play-off with Martin Kaymer and Bubba Watson.
However, the 31-year-old was then penalised two strokes after it was ruled he had grounded his club in a fairway bunker before hitting his second shot, even though spectators had been trampling all over the hazard.
Johnson had also squandered a three-shot lead in the final round of the US Open two months earlier and three-putted the 72nd hole at Chambers Bay this year to finish a shot behind Spieth.
The world number eight, who led after the first and second rounds of the Open at St Andrews, said: "I really felt like I had my ball under control a lot like I did at the US Open. We'll just have to see what happens. It's only the first round, we've still got a lot of golf to play."