Ex-Formula 1 world champion Mario Andretti says Fernando Alonso has a “real chance” of winning the Indy 500 on his debut.
Alonso, 35, will miss this year’s Monaco Grand Prix for the 500-mile race.
The two-time world champion said his first experience of Indianapolis was “fun” as he began testing on Wednesday.
Former IndyCar champion Andretti, 77, said: “His chances are real of potentially winning this thing.”
The 1978 F1 world champion, who runs the team Alonso is driving for and is the father of ex-F1 and IndyCar driver Michael Andretti, said this is a “golden opportunity” for the Spaniard.
“He’s at the top of his game and he doesn’t have too much to lose in Formula 1,” Andretti told BBC Radio 5 live.
“He can give Monaco up and give this a good try and maybe come away with a very happy result.”
Alonso, who won the Monaco Grand Prix in 2006 and 2007, said he had long held an ambition to win the so-called ‘triple crown’ of Monaco, the Indy 500 and Le Mans.
Only one man has won all three in his career – the late Graham Hill in the 1960s.
Alonso ended his test with a fastest lap of 222.548mph. Last year’s pole position time for the Indy 500 was 230.760mph.
American Alexander Rossi won last year’s event to become the first driver to win the race on his debut since 2001.
“Everything went like he [Alonso] has been there before [in testing]. He’s probably come away very pleased with himself and the team are very pleased with him,” added Andretti.
“I just feel very good for him, I’m very confident that it’s going to be a great experience overall.
“He will have no problem and have some fun with it.”