Toby Alderweireld knows as well as anyone just how tough a challenge high-flying Saints will face tomorrow in Diego Costa.

Alderweireld was teammates with the Chelsea striker last season, with both men part of the Atletico Madrid side that won La Liga before nearly lifting the Champions League.

They will be rivals tomorrow, though, as Ronald Koeman’s men host Jose Mourinho’s league leaders at St Mary’s in a top-four clash.

Alderweireld will likely be one of the men assigned to marking Spain international Costa, and he knows how tough a task shackling the £32m hitman will be.

“It is very difficult, because he is fast, he is strong, he has technique,” said the on-loan Saints star.

“He can score goals, is good with his head, so I have to be at my best to stop him.”

Costa, who has 13 Premier League goals this season, will not be the only Chelsea player who Alderweireld will face a reunion with tomorrow.

He also knows Filipe Luis from his parent club, Atletico, as well as Thibaut Courtois and Eden Hazard from the Belgium international team.

“They are some of the best players in the world, especially Diego and Eden Hazard,” said Alderweireld.

“We have to be at our top to beat them.”

Saints have been close to that level in their last two games, beating Everton 3-0 before dominating Crystal Palace 3-1 yesterday.

Alderweireld bagged the third at Selhurst Park, with a thumping header from a James Ward-Prowse corner, marking his first goal for the club and capping another magnificent display from the 25-year-old.

“It’s the best feeling, I think, even for a defender,” he said.

“I am very, very pleased that I could help the team and after we made it 3-0 it was over, I think.”

Alderweireld added: “It is very important after a couple of difficult weeks to win again.

“They were two difficult games, Everton and now here at Crystal Palace, but we got six out of six points, made six goals and conceded one.

“We are getting our confidence back.”

That could be vital, as Saints – who have returned to fourth place – head into a run that sees them face Chelsea, Arsenal and Manchester United in their next three Premier League games.

Unlike the last run of fixtures against the division’s established elite sides a few weeks ago, they might have key players coming back into their team this time – rather than going out of it.

Alderweireld said: “In one or two or three games we lost everybody, you know?

“Now, everybody is fit, we have a good bench, so it helps in a game that we can make a change when it is needed.”

On the quick turnaround between Palace and Chelsea, Alderweireld said: “Two days after a game, play another game – that has never happened to me before.

“I will prepare myself the best way I can and I hope to be 100 per cent for Sunday.”