The phrase "a significant competitor" is correct and usable in written English.
You can use it when discussing a business or market context to refer to a rival that has a considerable impact or presence.
Example: "In the tech industry, Company X has emerged as a significant competitor, challenging the market share of established players."
Alternatives: "a major rival" or "a key competitor."
Its entire marketing campaign depended on how close a significant competitor was to coming out with an equivalent product.
The big broadcasters, like Clear Channel and CBS Radio, say they do not see Pandora as a significant competitor.
The music companies EMI and Universal are participating in Amazon's store, making the service a significant competitor to Apple's iTunes service.
Scott rejected criticism that the ABC had become too big and was a significant competitor to struggling commercial media companies such as News Corp Australia and Fairfax Media.
The A.D.B. has reason to be anxious, he said, because the $50 billion in capital planned by China would immediately make the new bank a significant competitor.
"Universal's arrogance has paid off for them... they have destroyed a significant competitor, and that even with these divestments their ability to dominate and control the market has reached even more unacceptable levels," said Martin Mills, chairman of Beggars.
A world without Google would allow Facebook to remain dominant and unchallenged by any significant competitor in the social space.
The value and length of the deal with Qatar should help to make Glorious Goodwood in late July a more significant competitor for both Ascot, which stages the Royal meeting in mid-June, and York, which hosts the Ebor Festival in mid-August.
The app will allow users to stream Spotify's music to their handsets, which is surprising because the online service is seen as a potentially significant competitor to Apple's successful iTunes music store.
It is in the process of completing (awaiting merger approval) of an acquisition of its most significant competitor – the thermostat division of Borg Warner, a Fortune 500 auto parts supplier.
Amazon Japan's new focus on streaming video comes after competitor Rakuten took several big steps to position its $16 billion Internet services ecosystem as a significant worldwide competitor to Amazon and Netflix.
