Antonio Pizzonia had a Formula One drive with Williams almost sewn up until Germany's Nick Heidfeld came into the picture, Frank Williams said on Thursday.
At a news conference to announce a sponsorship deal with the Royal Bank of Scotland, Williams said that his team now needed more time to decide between their Brazilian test driver and his experienced rival.
Williams said both could expect contracts - "whoever loses will still be offered a test drive" - but a decision could be some weeks away.
"Nick's arrival into the testing scene was not foreseen," said the team boss. "We were very comfortable with having Antonio for 2005 but we felt it would be sensible to look at a rival in case there was someone better out there.
"If Nick had not been so obvious to us then Antonio would have been confirmed already, we think he's that good."
Pizzonia competed in four Grands Prix for Williams last season as a stand-in for injured Ralf Schumacher while Heidfeld made a strong impression for underfunded Jordan after losing his position at Sauber.
"We've decided that to be fair to the team and also to the two drivers to give them more time to demonstrate their abilities," said Williams. "I think we are talking two or three more tests. We'd like to have a decision certainly at the latest by the launch of the new car at the end of this month in Valencia."
The season starts in Australia on March 6.
Pizzonia was dropped by Jaguar after 11 races of his rookie season of 2003 after failing to score a point and being completely overshadowed by Mark Webber. Webber has since joined Williams and has little respect for the 24-year-old, who has claimed that the Australian got preferential treatment from Jaguar.
Although Williams say their choice will come down to which driver is the fastest, Heidfeld would be a more harmonious choice and he can also count on strong backing from engine partner BMW.
"I think Mark might be a little bit uncomfortable but if Antonio is the quicker driver we have to do what's best for the team," said Williams. "It'll be a factor in our discussions and final judgement I guess."
Pizzonia however has the support of Williams' Brazilian oil partner Petrobras.
Williams said BMW had convinced him and co-owner Patrick Head to give Heidfeld a chance: "BMW said 'You really ought to have a look at this guy, we think he might be good.'
"He is very quick so that is why we are putting ourselves through this testing programme.
"I hope we can have some sort of certainty about which driver is quicker. At the end of the day if they are both the same we'll have to toss a coin, I don't know. They're quite different drivers but very evenly matched in many respects. We'll see."
Williams finished last season in fourth place and have an all-new lineup this year after the departure of Colombian Juan Pablo Montoya and Ralf Schumacher.