Butch Harmon rates Padraig Harrington as one of the leading threats to Tiger Woods defence of the US Open at Olympia Fields near Chicago.
No European has won the title since Tony Jacklin in 1970, but Harrington believes he has learnt a lot from partnering Woods in the last two US Opens and beat him head-to-head for a million dollars in the Target World Challenge in California in December.
"Padraig's been able to look Tiger in the eye. He's a tough competitor and that's what it takes," said Harmon, who continues to work occasionally with Woods.
If Woods fails to win come would not be in possession of any one of golf's major titles.
Els, Rich Beem and Mike Weir are the other current holders of the majors and Harmon, while still rating Woods "far and away the best player in the world", thinks that the days of total domination may be over.
"It's not so much a case of Tiger slipping, more that the other players are raising their games."
Woods is out to show them, however, that on the US Open stage at least they need to find at least one more level to match him.
A victory on Sunday would put him in elite company. Only Ben Hogan and Willie Anderson have won the US Open three times in four years.
"I think it's everyone's hardest test," Woods said. "You have to drive the ball well there. You have to hit your irons well and you have to make a lot of eight-footers for par.
"It's the most physical and mental test we play. There's so much stress on every shot."