“It was funny seeing him there!” Tien said afterwards. “We have been in each other’s boxes a few times in the past. We knew that we’d play each other if I won. We’ve been practising together a lot in the last month. I told him I had a really good gut feeling that we’d play each other here!”

Tien is also now the youngest American man to record a win over an opponent ranked in the Top 20 since Ryan Harrison beat Ivan Ljubicic at the 2010 US Open.

“I thought I played really well today. It was great to rebound and have a good match after getting chopped by Joao yesterday. It was great to come out here and play good tennis, I thought I maintained my level pretty well from the beginning to the end.”

Fils picked up a warning for racquet abuse when he smashed it up with frustration after the fourth game of the second set.

He got himself back into it in the third set, stretching it to a tiebreak to take it 4-3 (4), but Tien took the decisive breaker in the fourth, holding his nerve to reach the final four.

Fils was looking to reach his second consecutive semi-final at the event, having lost in last year’s final to Hamad Medjedovic.