MONTE CARLO, Monaco - Jenson Button topped the times for McLaren at the end of Thursday's two opening free-practice sessions ahead of this weekend's Monaco F1 Grand Prix.The 2009 World champion outpaced Romain Grosjean of Lotus by three-tenths of a second on a day during which showers upset the teams' running plans.SUNDAY’S SHOWPIECEAfter a warm and dry morning under blue skies, the rain arrived after only 20 minutes of the afternoon practice session and this meant that only those drivers who were on the track in that early dry window were able to clock improved times.Button was out in the opening spell and he clocked a best time of 1min15.746 to take the “overnight pole” ahead of Sunday's showpiece event in the Mediterranean principality.Felipe Massa and his Ferrari team mate Fernando Alonso were third and fourth quickest ahead of Pastor Maldonado who recorded his and Venezuela's first F1 victory for Williams at the Spanish GP two weeks earlier.TOO WET FOR FASTNico Rosberg of Mercedes, winner of his maiden race in China earlier this season, was sixth ahead of Mark Webber of Red Bull and Kamui Kobayashi of Sauber. Michael Schumacher, in the second Mercedes, was ninth ahead of compatriot and defending World champion Sebastian Vettel of Red Bull.Button's McLaren team mate Lewis Hamilton was down in 11th after being affected by the rain which eased off after 10 minutes before returning to leave the circuit too wet to allow for fast times.In the difficult conditions, it was surprising that none of the drivers ran into the barriers although Heikki Kovalainen, whose Renault engine blew up in the tunnel during the morning, slid off the circuit at Mirabeau in the afternoon when he found it difficult to find reverse gear and had to be rescued.Alonso was fastest in the earlier morning practice session for Ferrari as the field began to learn its way around the unforgiving circuit. For Button, it was an encouraging outcome, but he remained cautious about the immediate future following a roller-coaster start to the season in which five different drivers have won the opening five races."I think we have a very strong car," Button said, “and aerodynamically it is strong, but I don't think we are able to show it sometimes.”'DIFFICULT SOMETIMES'Having started the year as the pace-setting team, with Hamilton and Button locking out the front row of the grid in both Australia and Malaysia, the McLaren pair have scored only 10 points in the last two races as their rivals have collected more points -- and many of them by surprising drivers -- in this season of the unexpected.For Button, this was as much due to the need for the team to understand the Pirelli tyres as it was to do with outright pace. "If you look at Lewis' lap in qualifying at Barcelona and if you look at our race pace in China and Australia, it is there, but it is extracting it which is quite difficult sometimes."The thing we don't understand, with the tyres, is the difference of pace up and down the grid. Pastor (Maldonado) qualified 17th in Bahrain, then second in Spain, which got turned into pole. That is what is difficult to understand."Optimistic, but puzzled, Button conceded that the season had not unfolded as he expected, but added that the topsy-turvy nature of the results was both entertaining for the fans and also welcome in keeping the title race wide open.16 POINTS OFF"When different people keep winning and the championship leader finishes fifth it is not such a problem," he said. "To be fair I've only finished three races in the points and the last one wasn't really in the points."I've had a first, a second and a ninth and that puts me 16 points off the lead, which is nothing. It is amazing how few points everyone is getting this year. I think there have been 125 points up for grabs and the leader has 61."While Lotus and Mercedes expressed confidence in their potential for the weekend, it was a surprisingly different story for the Red Bull team with Webber admitting he feared that their bid for a third successive Monaco pole was doomed."It was not easy for us. We have some work to do in the next 24 hours.”2012 Monaco F1 GP second free practice session times1 Jenson Button (Britain) McLaren 1min15.746 2 Romain Grosjean (France) Lotus 1min16.138 3 Felipe Massa (Brazil) Ferrari 1min16.602 4 Fernando Alonso (Spain) Ferrari 1min16.661 5 Pastor Maldonado (Venezuela) Williams 1min16.820 6 Nico Rosberg (Germany) Mercedes 1min17.021 7 Mark Webber (Australia) Red Bull 1min17.148 8 Kamui Kobayashi (Japan) Sauber 1min17.153 9 Michael Schumacher (Germany) Mercedes 1min17.293 10 Sebastian Vettel (Germany) Red Bull 1min17.303 11 Lewis Hamilton (Britain) McLaren 1min17.375 12 Paul Di Resta (Britain) Force India 1min17.395 13 Bruno Senna (Brazil) Williams 1min17.655 14 Nico Huelkenberg (Germany) Force India 1min17.800 15 Sergio Perez (Mexico) Sauber 1min18.251 16 Vitaly Petrov (Russia) Caterham 1min18.440 17 Jean-Eric Vergne (France) Toro Rosso 1min18.522 18 Daniel Ricciardo (Australia) Toro Rosso 1min18.808 19 Kimi Raikkonen (Finland) Lotus 1min19.267 20 Timo Glock (Germany) Marussia 1min19.309 21 Heikki Kovalainen (Finland) Caterham 1min20.029 22 Charles Pic (France) Marussia 1min20.240 23 Pedro de la Rosa (Spain) HRT 1min20.631 24 Narain Karthikeyan (India) HRT 1min20.886 Stay with Wheels24 for the Monaco Grand Prix weekend!