Wesley So downed world No. 1 Magnus Carlsen of Norway to win the Skilling Open championship on Monday.

So, who represented the Philippines before bolting for the United States chess federation, needed a playoff to beat Carlsen on the Norwegian world champion's birthday.

"First of all I’d like to apologize to Magnus for semi-ruining his birthday, but I’ve got to try to win from time to time, because we don’t want him to win 10 tournaments in a row," So said in an interview on Chess24.com.

• Wesley So: 'I’d like to apologise to Magnus for semi-ruining his birthday!'

So took home the $30,000 top prize offered in the online rapid tournament, which kicked off the $1.5-million Champions Chess Tour.

This isn't the first time So upended Carlsen.

The Cavite native defeated Carlsen at the World Fischer Random Chess Championship in November 2019, in Carlsen's home country.

• Wesley So, the Filipinos' loss is the Americans' gain

The first day Sunday of the 2-day final saw So and Carlsen split the 4 games on tap, with the white winning every time.

That changed Monday when Carlsen bagged the first game on black, before So did the same, according to Chess24.com.

They drew the next 2 games to arrange a playoff, where So won the first of 3 games and the second ended in a draw giving the American the title.

"I’m very shocked, of course, to beat Magnus," So said.

"He’s the best player in the world, so coming into the match all I wanted to do was to put up a good fight and make the games interesting, but today has been my day."

En route to the title, So downed Teimour Radjabov of Azerbaijan in the quarterfinals, and fellow American Hikaru Nakamura in the semifinals.