Juan Martin del Potro: I didn't expect to be in second week of US Open

Del Potro cruised past Spain's David Ferrer 7-6 (7/3) 6-2 6-3 to make his first major fourth round since Wimbledon three years ago.

The Argentinian has endured a nightmare spell since then with injury, after a chronic wrist problem meant he missed nine grand slams in a row.

Returning at Wimbledon this year, Del Potro is finally hitting the sort of form that saw him become world number four and a champion at Flushing Meadows in 2009.

He clinched a silver medal at the Olympics last month, losing to Andy Murray in a brutal final, and now faces Austrian Dominic Thiem for a quarter-final spot in New York.

"I didn't expect to be in the second week of this tournament," Del Potro said.

"I didn't expect to get a medal in Rio. I didn't expect to play at this level in this part of the year. Of course, I just want to be free with my wrist, trying to play in a good level as I did couple of years ago but I think I'm surprised with my level at the moment. I'm surprising the guys on tour, my colleagues. That's great because I'm seeing I can be dangerous in the future once again."

Thiem made the French Open semi-finals in May and is a rising star, having beaten both Roger Federer and Rafael Nadal already this year.

The 23-year-old came from behind to oust Pablo Carreno Busta 1-6 6-4 6-4 7-5, and Del Potro believes his opponent starts as favourite.

"I don't consider myself favourite. He could be the favourite, for sure," Del Potro said.

"But it all depends on my physicality, on my body, on my game. If I play like this today, I will maybe have chance to do a great match against him.