Kei Nishikori's comeback surprised the former world number four, who survived a major scare to advance at the Barcelona Open.

Nishikori trailed 6-4 4-2 and was forced to break serve as Guido Pella attempted to close out the match in the second set, before sensationally rallying to a 4-6 7-6 (7-4) 6-2 win on Monday.

A two-time champion at the ATP 500 tournament on the Barcelona clay, Nishikori broke serve on three occasions in the final set en route to the second round.

"He had the match today at 6-5, serving [for the match]," said Nishikori. "He was much better in the first and second [sets] and 3-0 up in the tie-break. I don't know how I fought through. [In the] third set, I was playing much better.

"I was making so many unforced errors in the first and second [sets]. Maybe he got a little bit tired and I was playing strong in the third set. I want to take that to the next round… The way I fought today and the way I played in the third set was good."

Next up for 2014 US Open finalist Nishikori is 13th seed Cristian Garin.

Ilya Ivashka topped Tallon Griekspoor 6-3 6-1 and the qualifier's reward is a showdown with top seed and 20-time grand slam champion Rafael Nadal.

Cameron Norrie, Pablo Andujar, Jeremy Chardy, Pierre-Hugues Herbert, Jaume Munar, Federico Gaio, Egor Gerasimov and Bernabe Zapata Miralles all advanced to the last 32.

At the Serbia Open, seeds Filip Krajinovic and John Millman booked their spots in the round of 16.

With the ATP 250 tournament being staged for the first time since 2012 and headlined by world number one Novak Djokovic, Serbian fifth seed Krajinovic overcame Stefano Travaglia 7-5 5-7 6-3 in a Belgrade rollercoaster.

Millman – the Australian seventh seed – took down local wildcard Danilo Petrovic 7-5 6-4 as Marco Cecchinato and Nikola Milojevic won through.