Australia's Caleb Ewan won a second stage of the Tour Down Under on Friday as Richie Porte's overall race lead was clipped to just three seconds by defending champion Daryl Impey.

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Ewan, who won Wednesday's second stage for Belgian team Lotto Soudal to lead the general classification for a day, sprinted past Irish first stage winner Sam Bennett to win the fourth stage of the UCI season-opening tour at Murray Bridge.

Belgian Jasper Philipsen (UAE Team Emirates) was third ahead of Germany's two-time TDU winner Andre Greipel.

"It was a very tough corner before the finish," Ewan said.

"I just told my last man to put me on the wheel of Sam Bennett, and that's what he did. That's exactly where I wanted to be on that last corner."

Australia's Porte (Trek-Segafredo) struggled in the crosswinds late in the stage but finished safely in the bunch and maintained his overall lead, although it was cut in half by Impey (Mitchelton-Scott).

"Days like this there's so much to lose. It was a hectic last 60 kilometres. It was a good day to get through unscathed," the 2017 TDU winner Porte said.

"We expected that Daryl would go for the time bonuses. To still have the (leader's) jersey going into tomorrow is a great feeling.

"Tomorrow in Victor Harbor, you never know what is going to happen with the wind, but I think we did a great job today. I think we are in a good position."

South African Impey, seeking three consecutive tour wins in Australia, claimed three crucial bonus seconds in the intermediate sprints and moved to within three seconds of the race leader.

Mitchelton-Scott controlled the first 40kms of racing in an attempt to set Impey up for intermediate bonus seconds to reduce Porte's GC lead heading into the weekend's final two stages.

"We will go for everything we can. We know it's going to be hard to beat Richie on Sunday," Impey said.

"We will need a buffer to try to win the race overall, so any chance we can get, we are going to go for it and we did that today. We will keep going for it, our tactics don't change."

Team Sunweb's Rob Power of Australia also gained a bonus second in an intermediate sprint and held on to third spot in the overall standings eight seconds down.