Sixth-seed Dominic Thiem produced one of his finest displays on clay to knock holder Novak Djokovic out of the French Open on Wednesday, but the Austrian said his "toughest opponent ever" awaits in the semi-final at Roland Garros.
Thiem faces fourth seed Rafa Nadal next with the Spaniard is in ominous form, having dropped just 22 games during his charge into the last four.
Thiem is the only player to have beaten Nadal on clay this season, but the 23-year-old is under no illusion about the size of the task that awaits him in Friday's semi.
"I mean, it's a joke how tough it is to win a slam," Thiem, told reporters after beating Djokovic 7-6(5) 6-3 6-0 in the quarter-finals.
"Because obviously now I beat Novak. On Friday is Nadal. In the finals there is another top star. That's why it's a slam because it's such a tough achievement."
Thiem, who plays with a single-handed backhand, said he needed to improve the defensive side of his game after struggling against the ferocious top-spin on Nadal's forehand during their last few encounters.
"I just have to watch that I'm not giving him his favourite positions on the forehand," he added. "I mean, it's one of the best shots I think ever in tennis. So anyway, you cannot avoid it all the time. I will concede some winners on Friday."
Thiem has put in some scintillating displays at Roland Garros this year and, like Nadal has yet to drop a set at the tournament.
"I like the conditions," he said. It's a fast clay court which fits my game very well."
Nadal reached the semi-finals after compatriot Pablo Carreno Busta retired due to injury in their quarter-final.
(The story was refiled to fixe a typo in Nadal in the eighth paragraph)
(Reporting by Hardik Vyas in Bengaluru; Editing by Ken Ferris)