Chelsea playmaker Nemanja Matic acknowledges Tottenham Hotspur are making life tough for Antonio Conte's side in the hunt for the English Premier League title.
The Blue side remain favourites to claim a second English Premier League title in three seasons, but have not raced away from the championship as they did in 2014-15.
Spurs' 2-0 victory over London rivals Arsenal on Sunday ensured the gap at the summit remained at four points with four matches remaining and Mauricio Pochettino's men play their next game at West Ham United on Friday – three days before the Stamford Bridge club host relegation-threatened Middlesbrough next Monday.
"I think this year is more difficult," said Matic, as quoted by The Mirror. "We finished 2015 eight points clear of Man City. I'm not sure this will be the case with Tottenham.
"I'm sure they will not give up until end of the season. At this moment it's hard, but we will see. Each time we win, they win. Two years ago that was not the case.
"The important thing is that we are playing well, we take points and we will see if they can drop some points."
"They have some difficult games, home and away, so everything is open until the end," the Serbia international added.