Javier Aquino was sent off in Tigres' 2-1 loss to Toluca. Azael Rodriguez/Getty Images

It was a rain-affected round of games this weekend in the Liga MX's 2018 Apertura, and one that saw Pumas and Cruz Azul remain undefeated and Chivas' problems rumble on.

Here are five talking points from the weekend:

1. Pumas, Cruz Azul remain unbeaten, top league

Mexico City clubs Pumas and Cruz Azul are one point clear at the top of Liga MX and the only undefeated teams after four rounds of matches.

Pumas and Pachuca played out a stale 0-0 draw in Estadio Olimpico Universitario, but Cruz Azul's 1-1 draw away at Tijuana made up for it in the entertainment stakes.

Cruz Azul went down to 10 players when Pablo Aguilar was unlucky to be sent off in the 22nd minute, but La Maquina played an intelligent game in largely containing Club Tijuana.

Xolos did, however, have chances to open the deadlock and eventually did when a Miller Bolanos header handed the home side the lead in the 79th minute, although there was some doubt about whether the ball had crossed the line after Jesus Corona made a spectacular attempt to keep it out. TV replays showed it probably just crossed the line.

Cruz Azul seemed doomed, but demonstrated resolve in picking itself up, attacking Xolos and finding a late equalizer via Gerardo Flores. It was a massive result, directly because the comeback went against every negative stereotype Cruz Azul has picked up over recent years as being a team that finds a way to mess things up.

This Apertura, Pedro Caixinha's Cruz Azul looks to have steely streak.

2. Chivas extend horrible home form

Chivas dropped a lead to lose 2-1 to Santos Laguna on Sunday in a defeat that will once again set alarm bells ringing. This wasn't a terrible performance, but a propensity to spawn chances and make mistakes at the back is a dangerous combination at any level and it was present again against Santos.

It's now one win in 19 Liga MX matches at home in the last 13 months for Chivas. Their last league win inside Estadio Chivas came on Oct. 28. Overall, the Guadalajara club has just seven wins in 38 games since July 2017. Here's the thing though: Over that time, no team has created more chances and are sixth in the expected goals table over that time. Finishing clearly remains an issue.

Coach Jose Cardozo must shoulder some of the blame in this game. The decision to take off right-winger Javier "Chofis" Lopez for holding midfielder Fernando Beltran at half-time was a play to control midfield better and the second half was less open, but it also robbed Chivas of their most dangerous player after the break.

Chivas are learning the hard way that it is difficult for a selling club to compete. The CONCACAF champion is now in 17th position on one point, held up only by Pachuca.

3. Tigres fail to break Liga MX record

Tigres were looking to make it a league record 30 consecutive home Liga MX games without defeat on Saturday and started on the right foot when a dominant opening was capped by Andre-Pierre Gignac finding the net with a trademark header in the 15th minute.

But just as it seemed Tigres would go on to school last season's finalist Toluca inside Estadio Universitario, a red card for holding midfielder Guido Pizarro in the 23rd minute for apparently throwing an elbow towards Luis Mendoza turned the game.

It was a contentious decision made all the worse in the eyes of Tigres fans because five minutes earlier Jorge Torres Nilo was bleeding profusely and required stitches after he was caught by Enrique Triverio's flailing arm.

Triverio -- who else? -- equalized in the 53rd minute and William da Silva gave Toluca the lead 10 minutes later, but Tigres had more possession even with a player down and could have scored themselves. When Javier Aquino was sent off for petulance towards the referee in the 70th minute, the game was all but over.

Now, with the loss to Cruz Azul last weekend, Tigres have lost back-to-back games for the first time since February 2017. There is no reason to panic -- the team is in eighth place -- but there are worrying signs.

For example, why did internationals Enner Valencia and Eduardo Vargas remain on the bench with Tigres chasing the game? Why can't Tuca Ferretti seem to find the right balance in terms of personnel in the defense? And what is the root of the recent indiscipline?

Ferretti has some thinking and work to do ahead of next week's game against Santos Laguna.

4. Statement win for Club America

Las Aguilas gave a demonstration of just why they are one of the top contenders for the title this Apertura in the 3-0 win over Monterrey on Saturday.

The poor state of the pitch in Estadio Azteca and a soft red card after 30 minutes for Stefan Medina didn't exactly aid the game as a spectacle, but Club America put on an assured performance against a Rayados side still adapting to life under Diego Alonso.

That was evident in America's first goal. Alonso's sides like to play out from the back, but winger Jonathan Urretaviscaya played an erratic pass into midfield, which America pounced on, with Roger Martinez finding himself free to finish from the edge of the penalty area.

Monterrey lacked punch upfront with Rogelio Funes Mori -- who has only recently returned from injury -- left on the bench by Alonso.

America coach Miguel Herrera said afterwards that it could've been more than 3-0 and he wasn't wrong. For Monterrey, Alonso will want to see more from Pizarro and Aviles Hurtado in coming weeks and will need time for his ideas be replicated on the field.

5. Early reds spoil entertainment

Tigres against Toluca, America versus Monterrey and the Tijuana and Cruz Azul clash were arguably the three biggest games of the weekend. All three were potential playoff clashes and all three had red cards in the first half, which obviously altered the course of the games.

It's difficult for the officials when too many players are harassing referees, demanding cards and diving around trying to earn an advantage, but the referees also need to think of the spectator.

Over the past weekend, those watching were robbed of genuine high-quality match-ups when none of the three red cards -- Pizarro (Tigres), Pablo Aguilar (Cruz Azul) and Medina (Monterrey) -- were nailed-on sending offs.