Former world No. 1 Andy Roddick used the word "insane" to describe the level of tennis seen at this year's US Open. Despite extreme heat and humidity, the US Open so far has featured a few thrilling and memorable matches.

World No. 1 Rafael Nadal and Austrian rising star Dominic Thiem produced one of the best matches ever seen at Flushing Meadows when they met in the quarter-final. No. 1 seed Nadal edged out ninth-seeded Thiem 0-6 6-4 7-5 6-7 (4) 7-6 (5) in four hours and 49 minutes of action to reach the semi-final.

Both players played at a stunning level but eventually it was 17-time Grand Slam champion Nadal who came up big when needed most. Nadal and Thiem started their match on Tuesday night and ended it in the early hours of Wednesday.

A day before the Nadal versus Thiem match, world No. 55 John Millman produced one of the biggest US Open shocks ever when he beat Swiss Roger Federer. Millman, 28, handed a 3-6 7-5 7-6 (7) 7-6 (3) loss to record 20-time Grand Slam champion Federer in the quarter-final.

Defending champion Nadal is set to meet Juan Martin del Potro in the semi-final, while Novak Djokovic will be fighting against Kei Nishikori for a place in the final. "The level of tennis at this years US Open has been insane.

Props to all the players for continuing to push their limits. I can't wait to see how it all shakes out this weekend," Roddick wrote on Twitter. Also read: Mardy Fish reflects on epic Rafael Nadal vs Dominic Thiem US Open clash