The Toffees host league-leaders Chelsea, while Tottenham welcome rivals Arsenal looking to close the four-point gap to the top.
Antonio Conte's men came through a stern test against Southampton on Tuesday to maintain their lead and Gary Cahill called the victory a "massive step", but Koeman has warned the Blues the race is still on.
"It's not done," he said.
"We know the gap from second to first is four points and both have difficult fixtures. This weekend can be decisive in the title race, what happens on Sunday."
"The pressure is always for the teams who are behind first.
"Chelsea is still first in the league, they are strong, confident but they know it will be a diffcult game for them."
Eden Hazard scored twice as Everton were beaten 5-0 by Chelsea at Stamford Bridge in November and Koeman is keen to show the progress made by his players since then.
He added: "We have had only one game this season where we've had no chance, and that was the Chelsea away game.
"From the start until the end, no chance to get even one point out of the game. We like to change and show that the team is imporving, that the team is much better."