2009 Formula 1 World Drivers Champion Jenson Button has admitted that Fernando Alonso is "one of the drivers you worry about" as the former McLaren driver gears up to compete against his former team-mate at the 24 Hours of Le Mans next month.
Button and Alonso, who raced alongside one another at McLaren from 2015-2016 for two of the Woking-based team's three years with Honda power, will both compete in the iconic 24 hour race next month for SMP Racing and Toyota respectively.
With the drivers holding a combined tally of 47 grand prix wins and 147 podium finishes during their time in F1, the race at Le Mans will mark the first time that the pair will have raced against each other since the 2016 Abu Dhabi Grand Prix, with both drivers competing in the FIA World Endurance Championship's premier LMP1 category.
"He's definitely one of the drivers you worry about," Button told Sky Sports. "Whatever you're racing in, if he's competing – you're going to worry.
"He's not just quick. He's a very clever driver, he's got the experience and he is always there. If you have a great day, you might out-qualify him and you might have a great race but he will always be on your heels. He very rarely has a bad race, which is tough when you're team-mates with him.
"We're going to go into Le Mans racing against each other and I know if we're in a stint together, he's not going to put a foot wrong. It doesn't matter if it's 3 o'clock in the morning or 3 o'clock in the afternoon, he's going to be 100 per cent.
"You've got to keep up with that and it's tough but that's what I love about racing against the best drivers in the world. They keep you on your toes," he added.
Holding two victories at the Monaco Grand Prix and after contesting the Indianapolis 500 last year in a McLaren-backed Andretti Autosport entry, Alonso is aiming to secure the triple crown in motorsport, now looking to 13.626 km long Circuit de la Sarthe track where he aims to claim victory.
With Button also holding a victory in Monaco, which was the product of his championship year with privateer team Brawn GP, the Briton could near closer than Alonso to the triple crown if he ends up standing on the top step of the podium at Le Mans this June. Despite this, however, Button ruled out an entry in the Indy 500:
"I'm going to leave that one for Fernando to do. This is pretty much the busiest season I've had in my career," he told Sky Sports, competing in the GT500 category of the Super GT Championship for Team Kunimitsu.