(SportsNetwork.com) - Formula One is set to begin its 66th season, and with it a multitude of driver changes from last year, including Sebastian Vettel's move to Ferrari and Fernando Alonso's return to McLaren.

Lewis Hamilton is the reigning F1 world champion, and Mercedes is ready to defend its constructors' title after winning it for the first time in 2014. But Vettel's arrival at Ferrari has re-energized the Italian team in hopes of returning to its glory days following a dismal season last year. Vettel and Ferrari are likely to be Hamilton and Mercedes' biggest threat for the driver and constructors' titles.

Alonso departing Ferrari and heading back to McLaren where he drove during the 2007 season is another notable driver change for this season. Alonso, though, will miss this weekend's season-opening Australian Grand Prix after suffering a concussion during an accident in testing last month in Spain. He is expected to return for the Malaysian Grand Prix the last weekend in March.

Daniil Kvyat was promoted from Toro Rosso to Red Bull where he will fill the seat vacated by Vettel and team up with Daniel Ricciardo. Toro Rosso revamped its driver lineup with rookies Carlos Sainz Jr., the 2013 and '14 Formula Renault 3.5 Series champion, and Max Verstappen, who at the age of 17 is scheduled to become the youngest driver ever to make his F1 debut.

Following 12 days of preseason testing in Barcelona and Jerez, Spain, Ferrari and Mercedes look strong heading into Melbourne, Australia. Vettel and Kimi Raikkonen put Ferrari to the top of the time charts in three of the four days in Jerez. Hamilton and Nico Rosberg from Mercedes were among the quickest during the second and final round of testing in Barcelona.

Williams' teammates Felipe Massa and Valtteri Bottas as well as the Lotus duo of Romain Grosjean and Pastor Maldonado were also impressive in Barcelona.

While testing was merely a dress rehearsal for the 10 teams and 20 drivers on this year's F1 entry list, no one is quite certain who will perform the best throughout the year.

CAN VETTEL RETURN TO HIS WINNING WAYS?

Vettel and Red Bull won four straight driver and constructors' championships from 2010-13, but Vettel and the team went in the opposite direction last season, thanks in part to the new technical regulations. The cars were equipped with a new power unit, which consisted of a 1.6-liter, V6 turbocharged engine along with an Energy Recovery System (ERS).

Vettel scored a season-record-tying 13 victories in 2013 but failed to win a grand prix last year. The German appeared on the podium just four times and placed fifth in the world championship standings.

His move to Ferrari, replacing Alonso, is one of the biggest driver changes in F1 history.

Driving the SF15-T, Ferrari's car for the 2015 season, Vettel posted the quickest times in the first two days of testing in Jerez. He had a mishap on his initial day of testing in Barcelona, spinning out on the gravel as he struggled for grip. Vettel placed second on the time charts during the final test day.

"Over the winter, we managed to complete the majority of the program we had set ourselves," Vettel said. "Now it's time to take to the track in Australia and see just how competitive we are."

Ferrari has been the most successful constructor in F1, but last year's disappointing season prompted not only a driver change but one in management as well. Just after the season concluded, Maurizio Arrivabene replaced Marco Mattiacci as team principal. Ferrari did not win a race and finished fourth in the constructors' championship in '14. It was the first time the team was winless in a season since 1993.

Vettel is hopeful that he and Ferrari can return to the top podium, beginning with the Australian GP, and be strong contenders for the driver and constructors' titles this year.

"I would say that the path to the very top right now still leads via Mercedes, and it is our aim not only to draw level with them but to surpass them," he said. "That is a very ambitious goal. But let's stick to the moment. It is much too early to make any statement on how things will develop. We simply haven't seen enough yet to stick the head out. Starting the season well is the aim right now, and then let's take it from there."

WILL HAMILTON, MERCEDES REPEAT AS CHAMPIONS?

Mercedes had a banner season in '14, with Hamilton and Rosberg in a league of their own. Both drivers staged a thrilling tug of war for the world championship that was compared much to the fierce rivalry between McLaren teammates Ayrton Senna and Alain Prost in the late 1980s.

Hamilton won 11 of the 19 grand prix and finished 67 points ahead of Rosberg for his second F1 world championship. His first title happened in 2008 when he drove for McLaren.

Mercedes did have some reliability issues with its F1 W06 Hybrid car in preseason testing. Hamilton had to miss one of his test days due to an illness, while Rosberg dealt with a strained neck.

"At this point in the year, you don't really know what to feel as a driver," Hamilton said. "You're impatient to get back to racing but, at the same time, you never know what a new season will bring. Testing gives you a basic idea of whether the car feels good or not but, beyond that, it's anyone's game to win."

Rosberg won last year's Australian GP. He went on to score four more victories during the season. Rosberg also recorded 11 pole wins. He had been in a tight points battle with Hamilton until his disappointing 14th-place finish in the season-finale in Abu Dhabi. Hamilton won that race.

"2014 was a really incredible season for the team, and I know everyone is pushing harder than ever to do it all again," Rosberg said. "This will be my sixth year with the team, and we're getting stronger with every season ... We had a good winter, but it's never straightforward. You always have challenges to overcome, and there's nothing for certain, so we have to keep pushing."

EARLY TROUBLES FOR MCLAREN

Even though Alonso is back on board with the team, replacing Kevin Magnussen, McLaren has endured numerous problems during the offseason, particularly with its MP4-30.

Honda returned to F1 as an engine supplier this year, providing McLaren with a V6 engine and ERS package. McLaren had a 20-year partnership with Mercedes- Benz as its engine supplier.

McLaren dealt with a number of technical glitches during the Jerez test, as the car lacked pace. The team's two Barcelona tests were just as problematic as Jerez, this time hydraulic and sensor issues to blame. McLaren was at or near the bottom of the time sheets in almost all of the days of testing.

"It is obviously disappointing that we weren't able to do as many kilometers in testing as we had hoped, but we are undeterred and working relentlessly to improve the reliability of the MP4-30, and ultimately, make progress in terms of our raw pace," said Eric Boullier, the racing director for McLaren. "Despite our difficulties, our package shows a lot of promise."

McLaren has not won a grand prix since Brazil in 2012, with driver Jenson Button.

Magnussen, now the test and reserve driver for McLaren, will fill-in for Alonso in the Australian GP. Magnussen competed in 19 grand prix for the team last year. He finished second in his F1 debut in Australia.

"Of course, I'm very disappointed not to be joining Jenson in Melbourne in the McLaren-Honda garage, but I understand the recommendations of the doctors, and I'm already back in training and preparing for my first race in the MP4-30," Alonso said. "I wish Kevin all the best for this weekend in Australia, and I know the car is in his safe hands."

NEWCOMERS

This season will feature four rookies.

Felipe Nasr will make his F1 debut with Sauber in Australia. Nasr finished third in last year's GP2 Series point standings. He also served as Williams' test and reserve driver.

Sainz and Verstappen replaced Jean-Eric Vergne and Kvyat at Toro Rosso.

This week, Marussia named Roberto Merhi as the team's second driver for this season. Merhi had driven for Caterham in three practice sessions last year. Will Stevens was appointed as Marussia's first driver earlier this year. Stevens made his F1 debut four months ago in Abu Dhabi.