Is there an idiom for "winning a contest because you were the only participant"?

Is there an idiom for winning a contest because you are the only participant and there is no competitor?

Winner by default. This is a common expression in America for one-on-one sports like wrestling and tennis, especially when part of a larger tournament.

Suzanne won her match by default when the Deerfield High School bus broke down, leaving her opponent miles away from the meet.

This is known as a walkover.

It comes from horse racing where the sole contesting horse had still to walk over the course to win. The following is from Merriam-Webster:

Definition of walkover

1: a one-sided contest : an easy or uncontested victory

2: a horse race with only one starter

The earliest citations in the OED (1928) (there spelled “walk-over”) are of metaphoric use in an article in the ‘Times’ concerning a parliamentary election in 1838 (spelled “walk over”), and one relating to a horserace in 1861 (hyphenated spelling), although it is not entirely clear whether the latter use is literal or metaphoric. A clear example of literal use in relation to horse racing is in the Report of the California State Agricultural Society of 1885:

On Monday, the fifteenth, the track was in the best possible condition when the first race was called, which was the Maturity stake for four-year olds, a dash of three miles. For this William Boots’ colt Padre had a walkover—Lucky B, Gano, and Augusta E paying forfeit.

However race records from the 18th Century include the verbal use, “walked over”, e.g.

Eclipse ch c 1764 (Shakespeare or Marske - Spiletta, by Regulus)…

In 1769 he won a £50 Plate at Epsom…

Walked over for the King’s Plate at the same meeting.…

As regards contemporary use, in addition to horse racing, the term is employed in other sports, including tennis, athletics, and basketball and badminton.

I was trying to remember the phrase that I've heard most often (I knew it was "Winner of a... race.") when I saw the comment by Chris Chudziki under David's answer, which filled in the rest of the phrase:

Winner of a one-horse race.

Upon searching for that exact phrase, it seems that it is most often used as part of an insult:

"He couldn't pick the winner of a one-horse race."

But it is used in the manner you described, and I think most native English speakers (at least on the U.S. side of the pond) would recognize the phrase.