By Martyn Herman

LONDON (Reuters) - The lengthy scar on Juan Martin del Potro's left wrist serves as testimony to the misery the Argentine has endured since an epic Wimbledon semi-final against Novak Djokovic three years ago.

That brutal five-setter softened up eventual winner Djokovic for Andy Murray to take full advantage in the final.

But far worse was to follow for 2009 U.S. Open champion Del Potro who, since that sweltering day, had completed only four grand slam matches prior to arriving at the All England Club ranked 165th in the world.

A year ago Del Potro, 27, was undergoing a third operation on his left wrist -- the non-dominant one he uses to help him club his trademark double-handed backhand -- with his career in doubt.

So it was heartening to watch the six foot six inch (1.98-metre) former world number four in grand slam action for the first time in two and a half years at the All England Club on Tuesday, unless your name happened to be Stephane Robert.

The Frenchman was pummeled 6-1 7-5 6-0 as Del Potro, playing under a protected ranking, set up a heavyweight second-round clash against Swiss fourth seed Stan Wawrinka -- a match that in the past would have been on the second-week schedule.

For Del Potro, however, who is over the other side of the net hardly matters as his biggest opponent has been his body.

"My last two to three years have been too complicated and too tough," Del Potro, whose last grand slam match was at the 2014 Australian Open, told reporters.

"Today I am enjoying other things a lot more. One of the saddest things was not having my normal life, my routine."

He played only nine matches in 2014 and four last year, so the 19 he has managed so far this year, winning 12, represent progress, although it is still early days.

"To be honest, I feel my forehands and serves are working well at the moment," he said. "But my confidence is not there on my backhands yet, and that's tough when you play against the top guys. But my biggest challenge is to finish healthy and ready to make a good preparation for the next year."

Wrist injuries are the nightmare tennis players fear more than any other, with Rafael Nadal the latest to fall victim.

Even when physically repaired, the psychological scars can take even longer to heal, according to Del Potro.

"I'm working hard mentally because I have to deal with some pains and some frustrations when I want to hit a good backhands and I cannot do it," he said.

"This year will be more like a comeback on to tour, to feel the pressure of tennis. I hope to be ready for being an aggressive player next season.

"I just want to play free."

Del Potro, a gentle giant off the court, is a popular figure in the locker rooms and Wawrinka is pleased to see him back, even if he will be showing little sympathy on Thursday.

"For sure I think we all happy that he's back on the tour, hopefully without any injury, that he can play for long now," Wawrinka told reporters.

(Reporting by Martyn Herman; editing by Clare Lovell)