David Goffin celebrates after defeating Dominic Thiem at the ATP World Finals in London on Friday night. Photo: Alastair Grant/AP

LONDON – David Goffin admitted on Friday that he doesn’t “know what to do” against semi-final opponent Roger Federer at the ATP Finals.

Goffin dismantled fourth seed Dominic Thiem 6-4 6-1 to set up a last-four contest against the Swiss great on Saturday, but was not full of confidence about his chances at London’s O2 Arena.

“I’ve never found a key to beat Roger,” said the seventh seed, who earlier in the week launched his tournament in spectacular style by beating world number one Rafael Nadal in three gruelling sets.

“Honestly, I don’t know what to do tomorrow. But I’m going to try something, something different, something that I’ve never done in the past.”

The Belgian has never beaten 19-time Grand Slam champion Federer in six matches and most recently clashed with him in Basel last month, where he won just three games in a one-sided contest.

“I will try to do my best to play a better match than in Basel, for sure,” he told reporters at his post-match press conference.

“In Basel, it was not easy. He played well. He didn’t miss. He was really aggressive, as always. He returned so well. So it was not easy in Basel.

“I hope that tomorrow he’s not going to play the same match. But I will try something different for sure.”

The 26-year-old said he would have to change his game plan from the one he used against Nadal earlier in the week as he was facing six-time winner Federer on one of his favoured surfaces.

“Against Roger, on his best surface, it’s indoor,” said Goffin.

“It’s this surface, along with the grass. There is no wind. It’s tough to play higher, to find the loop when you play here on this kind of surface.

“It’s not easy. It’s perfect for Roger. But I will try, like I said, to do my best tomorrow to find some solutions, try to play my game, yeah, be aggressive. If he’s aggressive and he hits the ball really hard with his forehand, he’s dangerous.”