After graduating from Texas A&M University in August, Quah Jing Wen was reluctant to leave the United States, an environment that she had grown comfortable with over the past four years.

But the 20-year-old has adjusted well since returning to Singapore and was pleasantly surprised by the ease with which she has adapted to training here.

That showed yesterday as she rewrote national records in the women's 100m individual medley and 400m freestyle on the first day of the 9th Singapore National Swimming Championships (Short Course Metres).

At the OCBC Aquatic Centre, Quah first erased her elder sister Ting Wen's 100m IM record (1min 1.43sec) by clocking 1:00.94 in the heats, meeting the 'A' cut for the Dec 16-21 Fina Swimming World Championships 2021 (SCM) in Abu Dhabi.

The multiple SEA Games gold medallist then set a new mark with her time of 1:00.57 in the final.

In the women's 400m free heats, Quah clocked 4:08.37 to beat Rachel Tseng's time of 4:11:11.

She said: "My plan initially was to just try the training (in Singapore). To be honest, I was really reluctant to come back because after spending four years in one programme in the US, I got so comfortable doing the same thing.

"I knew what I was expecting and what I was getting so change was pretty hard for me to accept.

"I had the idea in my head that I was definitely going to go back (to the US), but when I did come back to Singapore to train, I was very surprised by how I was with the environment and the training programme and the people I'm surrounded with.

"I've found a programme that fits me for now and that shows in my first day of racing."

Also booking their spots in the World Championships were Teong Tzen Wei, Maximillian Ang and Glen Lim, who all set national records in their respective events.

Teong, 24, rewrote the men's 50m butterfly record twice.

The two-time SEA Games gold medallist first finished the heats in 22.32sec, erasing Joseph Schooling's previous record of 22.40sec, before lowering his time to 22.24sec in the final.

While his time in the final places him joint-ninth in the world rankings alongside Australian and multiple Olympic medallist Kyle Chalmers, Teong felt that there was still room for him to improve.

"I'm happy with the performance, but there are a lot of things to improve on and go faster," said Teong, who is still recovering from a torn shoulder labrum.

"I'm trying to keep the emotions in check, there are a few more days of racing so I have to put what I've done - good or bad - behind me and look forward to the next race."

In the men's 200m breaststroke, Ang rewrote his own record when he touched the wall in 2:08.49 to win the final, improving on his previous mark of 2:10.39, while Lim topped the men's 400m free final in 3:45.41, eclipsing Pang Sheng Jun's time of 3:48.55.

They both met the 'B' cuts for the World Championships.

Ang also qualified for the men's 100m IM.