After missing the last three rounds to try and shake off mystery fatigue problems, Casey Stoner made his eagerly awaited MotoGP return during Friday practice for the Portuguese Grand Prix at Estoril.

The 2007 world champion, who dropped from third to fourth in the championship during his absence, completed 19 laps, making three runs in total - his fastest time coming on his second run when he clocked a 1min 37.966, good enough for fourth.

Stoner took a short break before his final effort after struggling with arm pump, and the real test of his fitness will come in Sunday's race.

"So far so good," declared the double 2009 race winner, who took up his usual position as fastest Desmosedici rider from his very first flying lap. "It was great to be back out there on the bike today and I really enjoyed myself.

"Physically I feel about as well as you could expect considering that I haven't trained properly for almost four months now and I'm pleased with that. One thing I did struggle with was arm pump, which I expected, and I needed a bit of a rest in the later stages of the session. I expect the problem to ease over the weekend but I have to be careful not to push it too hard and exacerbate it further.

"We only did a couple of laps at the beginning but the riding position and brake levers weren't quite right, so I had to come in and change that. After that we were competitive and making constant improvements, until my final run when we made quite a big change to the setting and it didn't work out. If it wasn't for that the lap time could have been better.

"We have different swingarms on the two bikes but I only tried the carbon one today, so I'd like to try the aluminium one tomorrow and see where we go from there," continued Stoner, who start this weekend's event just seven points behind Dani Pedrosa .

"As far as the new fairing is concerned it was impossible to feel the difference today because it is designed to help the bike handle better in strong winds, which unusually for this circuit we didn't have today, and fast direction changes, which there are none of here," concluded Stoner, who was 0.781sec behind fastest rider Jorge Lorenzo (Yamaha).

Whilst Stoner used the new bodywork on each of his GP9 machines, with a carbon swingarm on one and an aluminium swingarm on the other, team-mate Nicky Hayden experimented with the new fairing on only one machine - but was unable to gauge its performance due to the unusually low winds.

The American used the original fairing on his other bike, with both of them fitted with the carbon swingarm. Hayden made four runs but left it late to set his best time, which came on his penultimate lap of 27, and was goo d enough for eleventh.

The 2006 world champion was 0.695sec slower than Stoner.

"It wasn't as good as we'd have liked it to be coming out of the gate - I'd thought the gap to the front would be closer than that today so we've got a lot of work to do," confessed Hayden.

"We need to make a general improvement everywhere - there's not one part of the track holding me back, I just need to find a little bit more speed in every section. We tried the new cowling in one run but it was just to get a bit of data and get an idea.

"We still haven't decided what way we're going to go with that for the rest of the weekend. It's still early but I would have liked to start out stronger and we need to pick it up a bit tomorrow.

"I also want to say congratulations to my chief mechanic "Bibo" [Davide Manfredi], whose wife just had a healthy baby boy."