In a session afflicted by several rain showers, drivers constantly switched between wet and intermediate rubber.
Mark Webber's Red Bull was the first to set a proper time, on wets, before people started to try Pirelli's inter tyre.
Jean-Eric Vergne's Toro Rosso and Felipe Massa then took it in turns to go quickest, with Vergne staying on top for some time with 1m42.155s, set on inters.
But, with the track at its best just after the halfway point in the session, Hamilton's Mercedes started a strong run of times on intermediates.
He set a string of 1m42s and recorded a best of 1m41.614s, which would remain the fastest of the session despite being around 20 seconds off a dry time.
Shortly after Hamilton's lap, Massa recorded 1m41.848s to end up second, ahead of Vergne.
Webber was often quickest when the rain was falling and ended the morning fourth fastest.
Two drivers were caught out by the tricky conditions and caused red flags. Romain Grosjean dropped his Lotus at Turn 12 before he had even completed a flying lap, while the Sauber of Esteban Gutierrez went off at Turn 3 just before the end of the session.
The F1 rookie had nevertheless completed more laps than anyone else and ended up sixth, behind Paul di Resta's Force India, but ahead of Grosjean.
Morning times (Driver/Team/Time/Gap/Laps)
1. Lewis Hamilton Mercedes 1m41.614s 43
2. Felipe Massa Ferrari 1m41.848s + 0.234s 40
3. Jean-Eric Vergne Toro Rosso 1m42.155s + 0.541s 38
4. Mark Webber Red Bull 1m43.649s + 2.035s 44
5. Paul di Resta Force India 1m44.444s + 2.830s 33
6. Esteban Gutierrez Sauber 1m45.020s + 3.406s 47
7. Romain Grosjean Lotus 1m45.147s + 3.533s 29
8. Valtteri Bottas Williams 1m45.457s + 3.843s 44
9. Charles Pic Caterham 1m47.521s + 5.907s 31
10. Sergio Perez McLaren 1m48.562s + 6.948s 23
11. Max Chilton Marussia 1m50.253s + 8.639s 34
- Motor Racing
- Sports & Recreation
- Felipe Massa
- Mark Webber