'Concerned' Schumacher wants more running
Veteran Nick Heidfeld seems poised to replace the injured Robert Kubica at Renault after setting the fastest time in testing at Jerez on Saturday.
Ferrari's Fernando Alonso was the long-time pace-setter but Heidfeld beat him by 0.132 seconds late in the afternoon.
German Heidfeld was being evaluated by Renault as a potential stand-in for Kubica, who suffered serious injuries in a rally crash last weekend.
Michael Schumacher's Mercedes was third ahead of McLaren's Lewis Hamilton.
Kubica is expected to miss the entire F1 season after suffering multiple fractures and a partially severed right hand in a crash on a rally in Italy last Sunday.
We had a good day, without too many problems, and luckily all the changes we made to the car went in the right direction and improved things, so I was able to push harder and harder
Although Renault have said they are also considering former Force India driver Vitantonio Liuzzi, and intend to give third driver Bruno Senna some time in the car on Sunday, they made it clear that if Heidfeld impressed them on Saturday, the drive was his.
And it seems highly unlikely that Heidfeld could miss out on the drive after such a performance - not least because he would rely on the team's backing to provide him with the car settings to be able to post such quick times.
Heidfeld, a veteran of 172 grands prix and a former team-mate of Kubica, wasted no time in proving his credentials. Within two hours of the start of his test, he had surpassed by 0.6secs the fastest time set by Renault number two Vitaly Petrov in two days in the car on Thursday and Friday.
Lap times are not necessarily directly comparable because of differing conditions. And Petrov, whose Friday was punctuated by spins, blamed his struggles on adapting to the Kers energy recovery and power-boost system, of which Heidfeld has experience from his time at BMW Sauber in 2009.
Team principal Eric Boullier said: "Our first feelings (about Heidfeld) are very positive: he worked well with the team, gave good feedback and improved the car during the day."
Heidfeld added: "I really enjoyed the day. It was fun driving the car, although it obviously takes time to get used to things when it's your first time in a car and working with a new team.
"We had a good day, without too many problems, and luckily all the changes we made to the car went in the right direction and improved things, so I was able to push harder and harder.
Alonso's presence in the Ferrari drew a crowd more akin to a grand prix
"In terms of the lap times, it's very difficult for me to compare with the other teams because we don't know the fuel levels or what the others were doing. My first impressions of the car were positive, although there are always areas where you can improve."
The early pace on Saturday was set by Kamui Kobayashi with 1:21.242, but Alonso supplanted him at the top of the times mid-morning when he dipped down to a 1:21.140 after 30 laps of running.
The Spanish double world champion, whose presence at the test drew a huge crowd and led to traffic jams on the way to the track, improved to a 1:20.966 on his next run and then 1:20.493 - just 0.1secs off the fastest time of the test so far, set by Schumacher on Friday.
Alonso's time stood until Heidfeld beat it with a 1:20.361 lap with 80 minutes of the test remaining. Heidfeld's time is 0.009secs slower than Schumacher's Friday mark.
World champion Vettel ended the day only sixth fastest overall, but his pace on longer runs appeared to underline the impression that the Red Bull is competitive.
It is very important to be able to do so many kilometres one after the other, without experiencing any problems
One insider at a top team told BBC Sport that while it was hard to be definitive about comparative pace because of the differences to lap times created by fuel loads and tyres, Ferrari appeared to have a slight advantage over Red Bull at the moment as the team with the fastest car.
Other sources agree that the Red Bull and Ferrari are the two quickest cars, but say the Red Bull has a significant speed advantage over longer runs.
Alonso, who managed 131 laps, said: "It is very important to be able to do so many kilometres one after the other, without experiencing any problems.
"It means we can pick up a lot of data which is useful especially when it comes to understanding the tyre behaviour, which is the biggest unknown at this test.
"We must not think about what the others are doing, in terms of times. It is better to focus on our own efforts and then we can see where we are in Bahrain in a month's time."
Hamilton was back in the McLaren after Jenson Button drove on Friday but after four hours had done only 16 laps, compared to Vettel's 61 and Alonso's 49.
World champion Vettel takes over the Red Bull, which looks the car to beat
The 2008 world champion had more success in the afternoon, setting a best of 1:21.099.
Scot Paul di Resta had his first taste of the new Force India which he will drive in his debut season and when the team stopped for lunch, he had managed a best of 1:22.945, then the ninth fastest time, after what
the team said on the social network site Twitter
was due to work on brakes, set-up and tyre evaluation.
Di Resta, who has been promoted from his role as reserve driver, did not improve on that time, and ended the day 10th fastest out of 11 runners after a trip into the gravel at Turn Nine.
The team said he had been "struggling with the brakes all day".
Brazilian veteran Rubens Barrichello took the Williams over from novice team-mate Pastor Maldonado, whose test ended early with a crash on Friday.
Barrichello said on
that his morning's testing had been halted by a Kers problem - the second in as many days for Williams.
He managed some running in the afternoon and ended up ninth fastest, behind the impressive Lotus of Heikki Kovalainen.
Lotus were the best of the three new teams last season but were about four seconds off the pace of the front-runners.
On Saturday, Kovalainen was just 1.350 secs off the pace.
Second pre-season test, day three, Jerez de la Frontera, fastest times:
1 Nick Heidfeld (Ger) Renault one minute 20.361 seconds
2 Fernando Alonso (Spa) Ferrari 1:20.493
3 Michael Schumacher (Ger) Mercedes 1:21.054
4 Lewis Hamilton (GB) McLaren-Mercedes 1:21.099
5 Kamui Kobayashi (Jpn) Sauber-Ferrari 1:21.242
6 Sebastian Vettel (Ger) Red Bull-Renault 1:21.574
7 Sebastien Buemi (Swi) Toro Rosso-Ferrari 1:21.681
8 Heikki Kovalainen (Fin) Lotus-Renault 1:21.711
9 Rubens Barrichello (Brz) Williams-Cosworth 1:22.227
10 Paul di Resta (GB) Force India-Mercedes 1:22.945
11 Jerome D'Ambrosio (Bel) Virgin-Cosworth 1:25.471