Pre-season testing came to a close in Bahrain today, Mercedes Lewis Hamilton edging out Williams Valtteri Bottas in the final moments.

The Mercedes driver was one of several to experience a mixed day but in the end it all came good as he posted the second best time of the week. Meanwhile, just moments after commenting that it was the only team not to have suffered a breakdown in pre-season testing, the Williams of Bottas ground to a halt on the main straight.

Indeed, there were so many stoppages today - seven in total - that the session was extended by twenty-five minutes, which also allowed organisers to runs the cars under the floodlights that will illuminate next month's Grand Prix.

Despite a nightmare start to his day, things improved sufficiently for Sebastian Vettel to complete 77 laps though the German's best lap was still over 4s off the pace. Meanwhile, it was another nightmare for Lotus where Romain Grosjean's session ended with flames pouring from the back of the E22.

Other than Vettel, who spun off at Turn 1 - now to be renamed in honour of his countryman Michael Schumacher - there were red flags for Grosjean (twice), Hulkenberg, Alonso, Bottas and Gutierrez.

The Mexican's stoppage brought a highly impressive run for Sauber to an end, the car having completed 91 laps this morning, with Adrian Sutil at the wheel, and a further 86 this afternoon courtesy of his teammate.

Other than Lotus, the other big disappointment of the day was at McLaren where Jenson Button completed only 22 laps due to an ICE (internal combustion engine) failure, similar to that which subsequently befell Bottas at Williams. Having dominated for much of pre-season testing these two late failures demonstrate that the Mercedes power unit is far from bullet proof.

Hamilton's running was delayed this morning due to a gearbox problem, the youngster not emerging for his first run until 12:40, completing five laps before the lunch break. A further 65 laps followed in the afternoon as part of a programme which included setup evaluation and qualifying simulation.

"This has definitely been the most challenging winter I've experienced and the car is still very much a work in progress," he said at the end of the day's running, "but we've learnt a lot over the course of these last few weeks and overall it's been a good winter of testing for us. There's been an incredible amount of work put in by the team back at the two factories and on track. I have to say a special thanks to the crew here today who had a tough task in front of them but never lost their cool. It was a tough start to the day but they managed to get the car turned around for me to complete some good laps in the afternoon, which was a fantastic effort.

"There's so much to learn with these new cars; it's just mind-blowing," he admitted, "and I don't think anybody can be fully ready for the challenge of this season. But I feel as ready as I can be and I'm looking forward to seeing where we are in Melbourne."

"It was another difficult start to the day," added Paddy Lowe, "having discovered a problem with the gearbox at around 04:00 this morning which then cascaded into a number of other faults and meant we didn't get out on track until 12:40. Great credit to the crew who I know must have been tired after an intense month-long testing programme, but still went the extra mile to get the car turned around before lunch. In the afternoon there were a lot of red flags which further hindered our progress but, with the session extended slightly, Lewis managed to get in some useful runs which brought our mileage total above any other team during this winter programme.

"It seems far longer than a month ago that we first rolled the car out on a cold, wet filming day at Silverstone for some very delicate running," he added. "It's been a long road to get to the stage we're at today, running reasonably reliably and showing potential in terms of pace, so all credit once again to everyone involved. There's still a lot to do before we get to Melbourne but we look forward to the challenge."

"We come to the end of our winter programme with plenty to think about heading to Melbourne," said Toto Wolff. "The first race is now less than two weeks away and it will inevitably be a reality check for every team on the grid. We have experienced ups and downs during these pre-season tests, with two challenging days in particular right at the end.

"The problems we have faced during the last 48 hours would have had a significant impact on a race weekend and reliability is still the biggest obstacle we have to overcome. With that in mind, we will be taking nothing for granted in our preparations for the first race and beyond. However, before we round off winter testing, I must pay a big compliment to our teams at Brackley and Brixworth; they have delivered a very promising platform to work from this year. Now we want to step things up to the next level to be successful this season.