Dani Pedrosa led opening free practice for the Czech Republic Grand Prix at Brno despite a scare earlier in the red-flagged session.
In the absence of Casey Stoner, the championship has been reduced to a straight battle between Jorge Lorenzo and Pedrosa, with Pedrosa leading the early laps at Brno.
But the Honda rider was seen limping in the pits after being hit in the left foot by stones kicked up by Valentino Rossi's rear wheel on the exit of the final turn.
Aspar's Aleix Espargaro was then clearly in pain after being struck on the hand by stones at the same turn shortly after, prompting red flags for a clear up.
The Repsol crew had crowded around Pedrosa to check his condition, but the Spaniard was fortunately able to return after the restart, when he retook the top spot from Lorenzo.
Pedrosa - who won his second race of the year last weekend at Indianapolis, to close to within 18 points of Lorenzo - finished the 45-minute session 0.146s quicker than Lorenzo's Yamaha.
Lorenzo was the first of four M1 riders in a row, with 2013 factory Ducati rider Andrea Dovizioso third (+0.186s), his Tech 3 team-mate Cal Crutchlow fourth (+0.415s) and factory rider Ben Spies fifth (0.443s).
The sixth and final rider within the 'lead group' was Gresini Honda's Alvaro Bautista, just under half a second from Pedrosa, with Rossi in seventh but 1.168s from the top.
Rossi's factory Ducati team-mate Nicky Hayden is absent this weekend due to his Indianapolis qualifying injuries.
Eight quickest this morning was Honda LCR rookie Stefan Bradl (+1.342s) followed by the leading CRT of Aspar Randy de Puniet (+1.664s).