Australia's Sam Welsford celebrated his birthday with a third stage win in another sprint finish as Mexican Isaac del Toro Romero clung on to the overall lead in Australia's Tour Down Under on Friday.

Welford (BORA-Hansgrohe) gave himself the ideal present on his 28th birthday, surging to victory in the 136.2-kilometre fourth stage into Port Elliott to make it three wins from three bunch sprint finishes in the WorldTour season opener outside of Adelaide.

The powerful Australian sped home ahead of Eritrea's Biniam Girmay (Intermarché-Wanty) and Dutchman Lars Boven (Alpecin-Deceuninck) to continue his super start to the season with his new German team.

While Welsford dominates the sprint points classification, Del Toro (UAE Team Emirates) finished in the peloton to hold on to the overall leader's ochre jersey, but only by one second over Girmay.

New Zealand's Corbin Strong (Israel Premier Tech) was a further one second away in third ahead of the weekend's make-or-break climbs up Willunga Hill and Mount Lofty to decide this year's Tour Down Under winner.

In a frenetic finish, Welsford found himself momentarily pushed back but surged through a gap to overtake Viviani and secure a remarkable win.

While Viviani fell short, settling for 19th place, Biniam Girmay of Intermarché-Wanty sprinted to second place, followed by Lars Boven of Alpecin-Deceuninck in third. 

As the Tour Down Under approaches its final stages, Welsford's stellar performance sets a high bar, promising more thrilling action in the days, and climbs, ahead.

Mexican Isaac del Toro remains in charge ahead of the climbs to come.

"That was such a fast run and I didn't know or how it would happen, but I just had a good run out of the corner," Welsford said.

"I wasn't directly on (teammate) Danny Van Poppel, but he knew where I was the whole time and I just used the guys in front of me as the lead out.

"This has been a perfect start to the year for us and really special to come into a team and have that backing straight away."

Welsford, who switched from Dutch team dsm-firmenich last year to join BORA-Hansgrohe, had to make his own run to the finish and zipped past Ineos Grenadiers pair Elia Viviani and Jhonatan Narvaez to win decisively.

Del Toro Romero was content not to take any risks in the fourth stage and stayed in the peloton to the finish, preserving his lead in the general classification.

"It was a sprint situation and I didn't want to take a lot of risk, I hope to stay good for the final two days," the 20-year-old Mexican said.

"It's a big climb tomorrow up Willunga Hill and I want to feel good, and I'm happy with that."

Girmay conceded that Welsford was too strong in the final sprint and was satisfied to finish second.

"I think I have to be happy with that. I couldn't push more than this. Today Sam was unbelievable," Girmay said.

"Sam is in his best shape, today I was in perfect position, but he was just stronger than everyone."

Australian Luke Plapp, coming off an Australian time-trial title victory and a third national consecutive road race win, pulled out of the tour before the fourth stage after injuries sustained in his high-speed crash in Thursday's third stage.

"After monitoring Plapp overnight, unfortunately, he will not start today's fourth stage of the Tour Down Under due to discomfort and ongoing pain from a large amount of wounds," his Jayco-AlUla team said.