In his latest BBC Sport column, Andy Murray discusses his blossoming partnership with Serena Williams in the mixed doubles and not playing against older brother Jamie at Wimbledon.

Playing alongside Serena at Wimbledon has been great fun so far and, as I joked after our first match, we are both younger siblings - so we are very competitive.

We want to have a few more days and matches playing together, hopefully until the final on Sunday.

I wouldn't say anything has particularly surprised me about Serena as a player or a person, even though I didn't know her particularly well before we played and we have not spent loads of time together.

But you expect someone who has been as successful as she has to be extremely competitive - and she is.

Even though it is mixed doubles, and her priority is singles, she goes out there and wants to win every point.

Her record speaks for itself and the more matches she plays, her performances and level will only increase.

We played better in our second-round match against 14th seeds Fabrice Martin and Raquel Atawo than we did in the first round, and I'm sure we will continue to improve as we get more matches under our belts.

We have put pressure on our opponents in all four of the sets we have played and created a lot of break-point chances, and I think as the matches go on we will get better at converting them.

What I think has worked particularly well so far is our returning games. Some of Serena's returning - especially in our second match - was brilliant.

Martin is a really tall guy and was serving huge, but she put a lot of returns in play on the first serve and then she took big cuts at the returns on the second serve and was making a lot of them.

That is not an easy thing to do and she won't have been used to returning serves of that size. And if you think how little she has been playing, the way she is picking that up is special. It's not something everyone can do.

'Wimbledon has been more relaxed this year'

Only playing in the doubles at Wimbledon is obviously different to playing in the singles and it has been a new experience for me. The tournament has been more relaxed.

I've enjoyed Wimbledon every time I've had the opportunity to play - it is just a little bit more stressful when you're playing singles. When it is doubles you are sharing that load together, which helps.

One of the things that is tough to get used to on court in mixed doubles is the difference between the man's serve and the woman's serve.

Martin's serve was getting up to 138mph and the ball is bouncing up around your head on the return. But when the woman is serving, it is a 30mph or 35mph difference and the ball is staying probably a metre to a metre and a half lower. That can be more challenging to keep the rhythm there.

However, the format is good fun and the crowd love to watch it.

You only see the mixed at the Slams really, but is a different format and one a lot of social tennis players play at local clubs around the country.

Serena and I shared a few jokes out there on Tuesday night - but we can't reveal what they were, unfortunately, because some of the language was not repeatable!

I was feeling more relaxed than the first match and that was probably because Serena was making me laugh out there, and hitting some amazing returns.

We are playing again on Wednesday and I'm looking forward to that. The crowds have been brilliant so far and, hopefully, we can put on another good display.

No tinge of disappointment that Jamie and I won't meet

Jamie losing in the mixed doubles earlier on Tuesday means we won't play against each other here at Wimbledon.

It is a shame he did not do as well in the men's doubles with Neal Skupski as he would have liked, and it is a shame I didn't either after losing in the second round with Pierre-Hugues Herbert.

But it is everyone else who talks about us playing each other. It's not like we are sitting there and texting each other, saying we could play each other in the third round of the men's, or the final of the mixed.

That is not how sport works. Wimbledon is one of the toughest tennis tournament in the world and the best players are here, so it's hard and we both know that.

Just because it is a good story for me to play against my brother it does not mean we are automatically gifted a passage through the draw.

Andy Murray was talking to BBC Sport's Jonathan Jurejko at Wimbledon.