Juan Martin Del Potro, the former US Open champion whose career has been damaged by the wrist injury which sidelined him in 2010, on Thursday revealed some of the obstacles to returning to his former prominence after reaching the semi-finals of the Dubai Open.
Del Potro, who has won no further Grand Slams and just seven titles since the career-threatening setback, kept alive his chances of an eighth by beating Daniel Brands, a surprising German qualifier, by 6-4, 6-2.
It was a big-serving battle of the two tallest men in the tournament, and for a while, until 4-4 in the first set it seemed that Brands might be capable of creating further upsets in the Gulf.
He beat two leading Frenchmen, Gael Monfils and Jeremy Chardy, while reaching the semi-finals in Doha last month.
However once Del Potro broke serve to snatch the first set, the match changed character completely with the Argentine getting right on top.
His greater concern then became the preservation of the still troublesome wrist.
"The good thing is it's not getting worse," he said.
"I can hit my backhand often day by day, and it's a good thing to be a hundred percent."
Del Potro then cast doubts on how certain that was. "But I need to be ready for tomorrow," he added. " And I will have the rest of the day to recover in my wrist and do the best what I can."
He concluded: "I know against (Novak) Djokovic tomorrow, or (Andreas) Seppi, I need to hit my backhand a hundred percent and then see if I have a little chance to win."
Djokovic, the world No.1 from Serbia, and Seppi, the seventh seeded Italian, were due to play their quarter-final in the evening session.
When Del Potro met Djokovic in the semi-finals of the ATP World Tour finals in London in November he won the first set and lost in three. "If I play him again it's going to be a very big chance for me," he said.
Later Tomas Berdych, the world number six from the Czech Republic, continued to cope well with a schedule which required him to travel 3,000 miles (4,800 kilometres) from Marseille where he contested the final on Sunday.
There he beat Dmitry Tursunov 6-2, 6-1 in the semi-finals. Now he beat the Russian wild card entry again, almost as solidly, by 6-3, 6-2. Berdych also looked more comfortable than earlier in the week.
"I am making the transition," Berdych said, referring to the change from cold European indoor courts to the warm and sometimes windy hard courts of the coastal resort here.
"Every day I have been getting better and better. I am getting better with more time on court, and am getting a good rhythm now."
This was Berdych's third match. His fourth, according to the seedings, was due to be against Roger Federer, the titleholder, who he has beaten five times, including their last meeting, in the quarter-finals of the US Open.
However the sport's most famous player was scheduled for a late evening quarter-final with Nikolay Davydenko, who beat Federer en route to winning the 2009 ATP World Tour finals.