Barcelona - Nico Hulkenberg ended Formula One's first pre-season test in Spain on a high for Renault on Thursday with the fastest lap so far by any of the new crop of cars.
The 31-year-old German, who holds the unwanted record of most races (156) without once stepping on the podium, lapped the Circuit de Catalunya in one minute 17.393 seconds in the late afternoon and on the softest tyre.
His car then stopped on the track, however.
Toro Rosso's Thai rookie Alexander Albon had led the lunchtime timesheets with a lap of 1:17.637 while Mercedes unlocked a little more pace and went faster than Ferrari.
British-born Albon, one of three Formula Two graduates making their F1 race debuts when the season starts in Australia on March 17, completed a whopping 136 laps - on top of 132 on Tuesday.
That seemed a good indication that the new Honda engine, now also used by Red Bull, had made good gains in both performance and reliability after the problems of recent years.
Albon's Russian team-mate Daniil Kvyat had been fastest overall on Wednesday while Red Bull's Pierre Gasly covered the second most ground with 148 laps.
Renault, who had said before testing that their engine had made a big step up over the winter, also showed good pace and Australian Daniel Ricciardo was second fastest on the morning timesheets.
They did less mileage than others, however, on Thursday.
Five-times world champion Lewis Hamilton was fifth, just behind Bottas who set his best lap on the softest and fastest tyres, but did his stint in the colder morning temperatures.
"In general, the car was feeling better today and is coming together. However, we still have to keep on digging deep, analysing the data and the changes that we are making," commented Hamilton.
"The reliability has been really solid which is a great showing of all the hard work that everyone in the factory has done."
Ferrari's Leclerc was sixth on harder tyres than any of those above him.
Alfa Romeo's Italian Antonio Giovinazzi did the most laps of all on Thursday (154), but also twice brought out the red flags, while British rookie George Russell managed the fewest (17) in the Williams.
Poland's Robert Kubica was also on track for Williams, who only started testing on Wednesday afternoon, and did 48 laps. He was still 10th and slowest, more than a second and a half off Canadian Lance Stroll for Racing Point.Reuters