The win allowed Barcelona to capitalize on Real Madrid and Atletico Madrid both dropping points over the weekend, pushing them from third place to the summit. Barcelona and Real Madrid are now level on 51 points, with Barca ahead on goal difference.

Atletico Madrid trail by a point, while Rayo Vallecano, who are in strong form, sit in sixth place with 35 points.

Barca did just enough to end Rayo's nine-match unbeaten run in the league, but the visitors were left frustrated by a number of controversial refereeing decisions.

A foul by Pathe Ciss on Inigo Martínez inside the box led to a penalty being awarded to the Blaugrana following a lengthy VAR review. Lewandowski calmly converted from the spot in the 28th minute to give his side the lead.

Although Barcelona controlled possession, Rayo Vallecano posed the greater threat, forcing Barca's goalkeeper, Wojciech Szczesny, to produce a stellar performance and deny them on multiple occasions.

Rayo were upset when their appeal for a penalty was rejected after Pau Cubarsi seemed to foul Ciss, a situation they believed mirrored the incident that led to Barcelona's penalty. Their frustration grew when Jorge de Frutos had a goal disallowed in the 42nd minute.

Martínez made a vital goalline clearance with his head just before halftime, while Szczesny's brilliance continued in the second half, helping secure the win for the Nou Camp outfit.

Rayo Vallecano should have equalized in injury time, but Jorge de Frutos missed a header from close range.

Flick's side are now on a 12-match unbeaten streak in all competitions, and the three points moved them back to the top of the LaLiga table, marking another twist in their rollercoaster season.

The Catalans had been 10 points clear in November, fell seven points behind in early February, and now lead Real Madrid on goal difference, with Atletico just one point behind.

"It's not good to always [look at] the table because there is a long way to go," Flick said about returning to the top spot. "I am happy for the fans, the club, they deserve this situation. We always said we fight to the end and it changes nothing. We have to fight until the end, match by match, this is how we want to finish the season.

"At the end we are happy with the win. A day like today, we are very happy to have the three points. It was very hard. It was not easy to win."

Flick confirmed that Jules Kounde was left out of the starting lineup shortly before the match after being late for a pre-match meeting.

This marks the second time this season that Kounde's tardiness has led to him being dropped. He was also benched against Alaves in October for the same reason.

Kounde eventually came on for Hector Fort in the 66th minute, with the Blaugrana manager stating that he will have a conversation with the French defender about his punctuality.

"Before the match, we have maybe two or three appointments for the players, meetings," Flick explained during the post-match press conference.

"And this is very important the players are there. It's not very difficult to do this. It's not only about me, it's about (respect) for the players, the club, the fans.

"I don't want to say more because I want to speak with him. But he was too late. It's a clear rule, so he cannot start. This is what happened today."

Robert Lewandowski penalty sees Barcelona edge Rayo Vallecano to go top of LaLiga